4 Discussion of individual stars

The following discussion is intended to highlight particularly interesting features in the light-curve behavior of individual stars in 1996/97 but is not supposed to be a summary of all the relevant literature data. For the more fundamental parameters of the stars in common with our previous APT paper (Strassmeier et al. [1997a]), we refer the reader to the discussion in that paper. For a complete listing of all available literature on a particular star we recommend to consult the SIMBAD data base at CDS Strasbourg.

Figure 3: As in Fig. 1 but for $\zeta$ And. The main cause for the light curve variability is the ellipticity effect

Figure 4: As in Fig. 1 but for HD 12545. Top row: Amadeus $V(RI)_{\rm C}$ data. Bottom row: Phoenix-10 UBV data

HD 123 (HR 5B, V640 Cas). This triple system consists of a visual binary with components A and B, where B is additionally a spectroscopic binary. The A component and one of the B components are visible in the optical spectrum. A subset of the photometric data in this paper was previously analysed by Weber & Strassmeier ([1998a]) along with radial velocities from high-resolution spectra. They found a weakly significant photometric period of 1.127 days and a radial-velocity period of 1.026 days which seemed to agree with the previously claimed period of 1.08 days (Brettman et al. [1983]). However, Griffin ([1998]) showed that the 1-f alias in the radial velocities presented by Weber & Strassmeier ([1998a]), i.e. a 39.5 day period, is the true orbital period of the Bab pair. In the present paper, we reanalyzed our Wolfgang by data but found no convincing single period despite that several equally strong frequencies for periods of 3.6, 9.5, and 16 days appear in the frequency spectrum in Fig. 1.

SAO 91772 (LN Peg). This is also BD+13$^\circ$13, and is a known spectroscopic binary with an orbital period of 1.844 days (Latham et al. [1988]). Henry et al. ([1995b]) identified it as a double-lined spectroscopic binary and determined the rotational broadening for both components (the specific values for $v\sin i$ were later revised by Fekel [1997] to 23.7 kms^-1 and 22.5: kms^-1 for the primary and secondary, respectively). Recently, Fekel et al. ([1999]) presented an orbit determination and summarized the star's fundamental properties. The light variability was discovered by Rodonó et al. ([1994]) who found a V amplitude of 0 $.\!\!^{\rm m}$08 and a period of 1.84 days, very close to the orbital period. Further photometry was presented and analysed by Henry et al. ([1995b]). Their data showed several possibilities including a 1.852 day period which seems to confirm the 1.84-day period found by Rodonó et al. ([1994]). Our data from the 1996/97 observing season show LN Peg with a small amplitude of no more than 0 $.\!\!^{\rm m}$04. The data in Fig. 2 also suggest a trend towards fainter brightness and even smaller amplitude by the beginning of the new observing season in May 1997. The periodogram shows two peaks, a stronger one at 0.5402 c/d (1.85 days), and its weaker 1-f alias at 0.4598 c/d (2.17 days). All data were phased with the 1.85-day period.

Figure 5: As in Fig. 1 but for VY Ari. The UBV data are from the Phoenix-10 APT

Figure 6: As in Fig. 1 but for V711 Tau. Top row: Amadeus $V(RI)_{\rm C}$ data. Bottom row: Wolfgang by data

Figure 7: As in Fig. 1 but for EI Eri

Figure 8: As in Fig. 1 but for the WTTS V410 Tau

HD 4502 ($\zeta$ And). Its spectral classification has been repeatedly given as K1II (Bidelman [1954]) but Strassmeier et al. ([1989]) pointed out that this is inconsistent with the radius from the rotational parameters and suggested a normal giant classification. The new Hipparcos parallax (d=55.6 $\pm$ 2.4 pc; ESA [1997]) results in an absolute visual magnitude of +0 $.\!\!^{\rm m}$35 $\pm$ 0 $.\!\!^{\rm m}$06, as expected for a K1III giant. Recently, Fekel et al. ([1999]) presented an orbit determination and summarized the star's fundamental properties. Previous photometry was summarized by Strassmeier et al. ([1989]) who found no direct signs of starspot activity from their photometry in 1983-86. The main contributor to the light curve variability is the ellipticity effect (Stebbins [1928]). Kaye et al. ([1995]) were able to subtract that effect from the photometry and recovered the spot wave with amplitudes between 0 $.\!\!^{\rm m}$04 and 0 $.\!\!^{\rm m}$003 for data taken between 1923 and 1993. Our data from the observing season 1996/97 show a significantly different appearance than those in 1983-86 (Fig. 52 in Strassmeier et al. [1989]). Most notably, the full amplitude is now 0 $.\!\!^{\rm m}$13 instead of 0 $.\!\!^{\rm m}$09, the two minima are unequal in depth, and the secondary maximum appears fainter than the primary maximum by 0 $.\!\!^{\rm m}$05 or 100% with respect to 1983-86. This confirms the starspot activity on $\zeta$ And.

HD 12545 (XX Tri). Besides the T Tauri star V410 Tau, HD 12545 is the most spotted star showing light amplitudes between 0 $.\!\!^{\rm m}$4 - 0 $.\!\!^{\rm m}$6 in V. The average V amplitude in 1996/97 was "only'' 0 $.\!\!^{\rm m}$3. With a maximum brightness of $\Delta V=+0\hbox{$.\!\!^{\rm m}$ }375$ in February 1997 (or $V = 8\hbox{$.\!\!^{\rm m}$ }170$ if we adopt the brightness for the comparison star from Strassmeier et al. [1997a]), HD 12545 appeared with its brightest magnitude ever observed since the discovery of its light variability in late 1985. Figure 4 shows the data, the periodogram, and the combined phase curve.

HD 17433 (VY Ari). Strassmeier & Bopp ([1992]) presented a photometric study of this star with data from 1974 through 1991 and for further references we refer the reader to that paper. HD 17433 is a single-lined spectroscopic binary with an orbital period of 13.2 days and an asynchronously rotating primary with a period of 16.4 days. The largest photometric variations were seen in 1989 with an amplitude of 0 $.\!\!^{\rm m}$28 in V. Our new data were taken with the Phoenix-10 APT and show an unambiguous period of 16.23 days with a maximum amplitude of 0 $.\!\!^{\rm m}$17. Figure 5 shows a very asymmetric light curve indicative of strong starspot activity.

HD 22468 (HR 1099, V711 Tau). This star is probably the most well observed spotted stars in the sky other than the Sun and is the target of, e.g., continuing Doppler imaging by several groups (e.g. Vogt et al. [1999]). It is a double-lined spectroscopic binary and consists of a K1 subgiant and a G5IV or V-IV star. Only the K1IV star is significantly active and spotted, and its light is modulated with the 2.8-day period. Our data in Fig. 6 are not corrected for the presence of the light of the G5 star and are therefore combined magnitudes. The true amplitude from the K1 star alone can be expected to be approximately 1.52-times larger than the observed combined amplitude. Also note that a third star, ADS 2644B, only 6 $^{\prime\prime}$ from V711 Tau, is within the 30 $^{\prime\prime}$ diaphragm of the Wolfgang-Amadeus APTs. Our new data were obtained with both Vienna APTs in VRI and by, the V and y data are shown separately in Fig. 6 along with the periodograms and the phase plots. The average seasonal amplitude in 1996/97 was 0 $.\!\!^{\rm m}$15 in V with a period of 2.837 days. HD 26337 (EI Eri). The current data are part of a larger program to continuously Doppler image this star throughout its spot activity cycle (Washüttl et al. [1999]). The combined literature and previous APT data revealed a 11-year sinusoidal light variation that may be interpreted as a spot cycle (Strassmeier et al. [1997a]). Further information is given in that paper. In the present paper, we analyse new VRI data from the Amadeus APT. The average seasonal period was 1.913 days and the maximum V amplitude only 0 $.\!\!^{\rm m}$09.

HD 283518 (V410 Tau). This is the star with the record photometric amplitude due to spots: 0 $.\!\!^{\rm m}$65 in V in 1994/95 (Strassmeier et al. [1997a]). During the 1996/97 observing season, its V amplitude was comparably small, the maximum peak-to-peak amplitude was 0 $.\!\!^{\rm m}$35 in November 1996 and approximately 0 $.\!\!^{\rm m}$3 for the rest of the season. The best-fit period was 1.872 days.

HR 1362 (EK Eri). In this paper, we just present the 1996/97 Amadeus data and show a plot of the V magnitudes versus time (Fig. 9). A complete analysis of much more photometry of this star, including the present data, was recently presented by Strassmeier et al. ([1999b]). The full amplitude in 1996/97 was 0 $.\!\!^{\rm m}$2 in V.

HD 283571 (RY Tau). RY Tau belongs to the class of classical T Tauri stars (CTTS) with irregular light variations. It is one of the few CTTS with sufficiently rapid rotation to make it a Doppler imaging candidate. Attention was drawn to it originally by Herbst & Stine ([1984]) after it had brightened by 2$^{\rm m}$ from 11th to 9th magnitude in 1983/84. Recently, Petrov et al. ([1999]) reported yet another brigthening by 1$^{\rm m}$ from 10 $.\!\!^{\rm m}$6 to 9 $.\!\!^{\rm m}$6 in late 1996. Neither of these papers nor Bouvier et al. ([1988]) found a periodic variability while Herbst et al. ([1987]) suggested possible periods of 5.6 and 66 days and Bouvier et al. ([1993]) found 24 days. Our periodogram indeed shows a peak at a frequency of 0.04 c/d (25 days) but we do not consider it significant according to our S/N=4 criterion. The large $v\sin i$ of RY Tau of 50 kms^-1 and the anticipated stellar radius of 1.8 $R_\odot$ (see Bouvier et al. [1993]) suggest a rotation period of around two days. Therefore, the origin of the 24-day period - if real - still remains to be determined.

In this paper, we present our VRI light curves from 1996/97 and show that the star had faded by 1 $.\!\!^{\rm m}$2 in V between February and March/April 1997. A preliminary report including also a V-I color curve was presented as a poster paper by Granzer & Strassmeier ([1998]). Our period analysis from a subset of the present data (excluding the times of the brightness drop) did not reveal a clear periodicity other than the nightly aliases (Fig. 10).

HD 283572 (V987 Tau). Just recently, Strassmeier & Rice ([1998b]) presented a Doppler image of this pre-main-sequence star for October 1997 (i.e. in the following observing season 1997/98) that revealed a large and very cool polar spot and numerous surface detail along its rim. In this paper, we present photometry for the 1996/97 observing season (Fig. 11) and complement the data already analysed in Strassmeier & Rice ([1998b]). The photometric period for 1996/97 was 1.529 days while the period from a subset of the 1997/98 data used by Strassmeier & Rice was 1.5495 days.

HD 283750 (V833 Tau). Data from three and a half observing seasons between 1992 and 1996 were presented in our previous APT paper (Strassmeier et al. [1997a]) which also contained a summary of previously available photometry of this star. This season's VRI data show a rapid brightness increase by 0 $.\!\!^{\rm m}$05 from the beginning of the observations in mid November until the end of December (Fig. 12). This change amounts to more than the amplitude due to rotational modulation of 0 $.\!\!^{\rm m}$03, which remained almost constant throughout the season. The photometric period in 1996/97 was 1.806 days.

HD 282624 (SU Aur). SU Aur is a relatively hot, classical T Tauri star and was proposed to have an inclined magnetic dipole with respect to the stellar rotation axis (Johns & Basri [1995]). A more recent study on the wind and accretion phenomena in SU Aur was presented by Unruh et al. ([1998]) who confirmed the phase lag between the wind signatures and the accretion signatures. It is not clear whether SU Aur exhibits periodic light variations due to rotation at all. Photometric periods between 1.55 and 3.4 days were reported by Herbst et al. ([1987]) and Bouvier et al. ([1988]) while periods from spectroscopic signatures range between 2.5 and 3.0 days (see Unruh et al. [1998]). A preliminary report on our photometry, including a V-I color curve, was presented as a poster by Granzer & Strassmeier ([1998]). Figure 13 shows the 1996/97 V data and the periodogram. No significant periods other than the nightly aliases were detected.

HD 31964 ($\epsilon$ Aur). $\epsilon$ Aur was the check star in another science group on Wolfgang and Amadeus, and here we present the Strömgren by data from Wolfgang in 1996/97. The star is the eclipsing binary with the longest known orbital period (27.1 years). The previous primary minimum occured in 1983 and the next will take place around 2009. No secondary eclipses were seen so far but might occur around 1997 $\pm$2 years. According to Carroll et al. ([1991]), the primary is a peculiar F0Ia supergiant and the secondary a close binary of two BV stars that are surrounded by a tilted disk with a central whole. Our by data in Fig. 14 show irregular, short-term, light variations with an amplitude of $\approx$0 $.\!\!^{\rm m}$01 in y but otherwise no evidence for a classical occultation or transit.

HD 31993 (V1192 Ori). For relevant references, we refer to our previous APT paper (Strassmeier et al. [1997a]). The data from the observing season 1996/97 verify the $\approx$28-day period found in that paper. The amplitude was still rather small, between 0 $.\!\!^{\rm m}$01 during December 1996 and 0 $.\!\!^{\rm m}$02 thereafter. The V-light curve in Fig. 15 suggests a downward trend of the mean brightness, opposite to what was seen during the previous observing season.

HD 33798 (V390 Aur). Fekel & Marschall ([1991]) presented a detailed study of this lithium rich and rapidly-rotating late-type star. As such its evolutionary state is highly unusual because all spectral information indicates that it is a single, evolved star of spectral classification G8III, and thus should have slowed down its rotation and depleted its surface lithium for a long time. Spurr & Hoff ([1987]) found the star to be variable with a 9.8-day period and an amplitude of 0 $.\!\!^{\rm m}$05 in V, that was confirmed by Hooten & Hall ([1990]) from additional data and interpreted as the rotation period of the star. Our new data were gathered with Amadeus in VRI and show an increasing overall brightness level within the 50-night observing interval from February to April 1997 (Fig. 16). The periodogram gives only a very weakly defined frequency at 0.1032 c/d (9.69 days) but is consistent with the period found by Spurr & Hoff ([1987]). The maximum peak-to-peak amplitude in V was 0 $.\!\!^{\rm m}$03 in late March.

HD 291095 (V1355 Ori). HD 291095 was detected in the ROSAT WFC all-sky survey and Cutispoto et al. ([1995]) discovered its light variability with a period of 3.82 days. At the time of their observations in late 1993, it showed one of the largest amplitudes of a spotted star (0 $.\!\!^{\rm m}$37 in V). Its colors and optical spectrum are consistent with a binary K1-2IV + G2V system (Cutispoto et al. [1995]; Osten & Saar [1998]) but a single K1-2IV classification can not be fully ruled out. Our 1996/97 data show the light curve with an amplitude of up to 0 $.\!\!^{\rm m}$25 in V and a period of 3.87 days (Fig. 17).

HD 43989 (V1358 Ori). Cutispoto ([1995]) discovered the light variability of HD 43989 with a period of 3.63 days after it was detected in the ROSAT WFC all-sky survey. The star was also included in a recent study on stellar physical properties by Osten & Saar ([1998]). From synthesis of the optical spectrum it was not possible to distinguish between a double subgiant (composite) spectrum with equal rotational broadening of 25 kms^-1 or a single subgiant with twice the rotational broadening ( $v\sin i\approx42$ kms^-1). During the 1996/97 observing season, the star was observed with both Vienna APTs in VRI and by. The number of individual data points was rather sparse and the separated light curves and periodograms did not show any obvious signs of periodic variability. Therefore, we decided to combine them by applying the transformation $y\rightarrow V$ from Olsen ([1983]). Figure 18 shows the combined y and V data. The total range of magnitudes was 0 $.\!\!^{\rm m}$05, usually already a moderately large amplitude, but no significant period was detected. The largest peak in the periodogram in Fig. 18 is at 0.305 c/d (3.28 days) and well below the S/N=4.0 criterium. Nevertheless, we adopt this frequency to phase the data but emphasize that it is accordingly uncertain and possibly even spurious.

Figure 9: V-band light curve for HR 1362 in 1996/97

Figure 10: As in Fig. 1 but for RY Tau. No significant period was found but the strongest peak at 25 days (f=0.04) is in agreement with the period found earlier by Bouvier et al. ([1993])

Figure 11: As in Fig. 1 but for HDE 283572. Top row: Amadeus $V(RI)_{\rm C}$ data. Bottom row: Wolfgang by data

Figure 12: As in Fig. 1 but for V833 Tau

Figure 13: Light curve and periodogram for SU Aur. No significant periodicity was found. See text

Figure 14: Light curve and periodogram for $\epsilon$ Aur. No significant periodicity was found. See text

Figure 15: As in Fig. 1 but for HD 31993

Figure 16: As in Fig. 1 but for HD 33798

Figure 17: As in Fig. 1 but for HDE 291095. Top row: Amadeus $V(RI)_{\rm C}$ data. Bottom row: Wolfgang by data

Figure 18: As in Fig. 1 but for HD 43989. The panels show the combined Wolfgang y data and Amadeus V data

Figure 19: As in Fig. 1 but for HD 51066

HD 51066 (CM Cam). A subset of the Amadeus data was recently used for the Doppler imaging efforts of Strassmeier et al. ([1998a]). HD 51066 is an interesting target because it is a single and rapidly-rotating giant. Attention was first drawn to it by W. Bidelman (private communication), and Henry et al. ([1995b]) presented photometric data from a full season in 1994 that showed the star with a period of 16.2 days and an amplitude of 0 $.\!\!^{\rm m}$05 in V. In 1996/97, HD 51066 showed an amplitude of between 0 $.\!\!^{\rm m}$01 and 0 $.\!\!^{\rm m}$02 in V throughout the entire observing season (Fig. 19) suggesting that its spot distribution was either relatively symmetric or its activity level very low. Our period analysis shows a 16.0-day period and thus confirms the 16.2-day period found earlier by Henry et al. ([1995b]).

Figure 20: As in Fig. 1 but for $\sigma$ Gem

Figure 21: As in Fig. 1 but for IL Hya

Figure 22: As in Fig. 1 but for HD 82443 All UBV data are from the Phoenix-10 APT

HD 62044 ($\sigma$ Gem). Henry et al. ([1995a]) summarized and analyzed all available photometry up to May 1992 and we refer the reader to this paper. Our new data is from the Wolfgang APT and was taken in Strömgren b and y. A maximum amplitude of 0 $.\!\!^{\rm m}$12 in y was seen in November-December 1996 that faded to approximately 0 $.\!\!^{\rm m}$08 by February 1997. Figure 20 shows the seasonal light curve, the periodogram, and the phased y light curve with a period of 19.615 days.

HD 81410 (IL Hya). Weber & Strassmeier ([1998b]) presented an extensive study of this star including our 1996/97 season's photometry and determined a first SB2 orbit. Recently, Fekel et al. ([1999]) presented an updated SB2 orbit and summarized the star's fundamental properties. In the present paper, we perform an independent period analysis for all three bandpasses and confirm the long-term average period used in our previous APT paper. Figure 21 shows the data, the periodogram, and the phased light curve with P=12.674 days. Just recently, Cutispoto ([1998]) published further UBVRI data from 1992.

HD 82443 (DX Leo). Henry et al. ([1995b]) found a photometric period of 5.43 days. A thorough discussion of the available literature is included in that paper. Strassmeier et al. ([1997a]) presented new APT data from 1994 and 1995 and found a long-term declining brightness by 0 $.\!\!^{\rm m}$1 between 1989 and 1996. Possibly, this is part of a cyclic behavior and further monitoring of HD 82443 is desired. In this paper, we present new UBV photometry from the Phoenix-10 APT that first show a decline of the amplitude until HJD 2 450 470 and then a rapid increase until the end of the observations (Fig. 22). The maximum amplitude of 0 $.\!\!^{\rm m}$10 in V was reached around 2 450 570. As a comparison the minimum amplitude near 2 450 470 was 0 $.\!\!^{\rm m}$035. The periodogram shows the strongest frequency at 0.1841 c/d (5.432 days), in excellent agreement with the period discovered by Henry et al. ([1995b]).

HD 82558 (LQ Hya). LQ Hya is a single, young and rapidly-rotating K2V star that attracted many studies in the past years. A recent Doppler image was presented by Rice & Strassmeier ([1998]) from CFHT observations in early 1995. Cutispoto ([1998]) published new UBVRI data from February 1992, and the star was also a target in our previous APT paper (Strassmeier et al. [1997a]). The long-term light curves in that paper revealed a sinusoidal modulation with a period of around 7 years. In the present paper, we add another full season of UBV and VRI photometry from the Amadeus and the Phoenix-10 APTs. The maximum V amplitude in 1996/97 was 0 $.\!\!^{\rm m}$06, thus twice as large as in the previous season but twice as small as in 1994/95. The best-fit period was for both data sets 1.60 days. The considerable scatter in the phase plot in Fig. 23 indicates a rapidly changing spot distribution.

Figure 23: As in Fig. 1 but for LQ Hya. Top row: Amadeus $V(RI)_{\rm C}$ data. Bottom row: Phoenix-10 UBV data

Figure 24: As in Fig. 1 but for $\xi$ UMa. Note that all detected frequencies in the periodogram in the right panel are aliases of the one-day observing interval

HD 98230 ($\xi$ UMa B). HD 98230 is the B component of the visual binary $\xi$ UMa (AB) and is only 2 $^{\prime\prime}$ away from A. Both visual components are also (single-lined) spectroscopic binaries but it is the primary of the B component (a G5V star) that shows strong Ca II H&K emission which is a measure of magnetic surface activity. The orbital periods for the A and B components are 669 and 3.98 days, respectively. For relevant literature data and references, we refer to the CABS catalog (Strassmeier et al. [1993]). Our differential photometry through a 30 $^{\prime\prime}$ diaphragm includes both spectroscopic binaries. Strassmeier et al. ([1989]) presented BV photometry from 1984 to 1986 that indicated a long-term variability of 0 $.\!\!^{\rm m}$05 in V and a possible periodicity of around 800 days. No short periods close to the orbital period of the B components were found. From the present data, we confirm the absence of a short period that could stem from rotational modulation (Fig. 24). A long-term trend is nevertheless obvious from Fig. 24; its observed amplitude amounts to $\approx$0 $.\!\!^{\rm m}$015 in y and agrees with the possible period of 800 days suggested by Strassmeier et al. ([1989]). HD 106225 (HU Vir). Cutispoto ([1998]) published further UBVRI data from 1992 that was not included in Strassmeier et al. ([1997a]). Hatzes ([1998]) updated and summarized the previous Doppler imaging efforts and Fekel et al. ([1999]) presented a new orbit determination and summarized the star's fundamental properties. For more information on this very active RS CVn star, we refer to any of these papers. Our new 1996/97 VRI data show the star with a variable light-curve amplitude ranging from 0 $.\!\!^{\rm m}$2 in December 1996 to 0 $.\!\!^{\rm m}$1 in March/April 1997 and back to 0 $.\!\!^{\rm m}$25 in June 1997 (see Fig. 25). The period from the more numerous Amadeus data was 10.66 $\pm$ 0.01 days and is confirmed by the Phoenix-10 data which yielded a more uncertain 10.60-day period. The "scatter'' in the phased light curves in Fig. 25 indicates the large degree of spot activity of this star.

Figure 25: As in Fig. 1 but for HD 106225. Top row: Amadeus $V(RI)_{\rm C}$ data. Bottom row: Phoenix-10 UBV data

Figure 26: As in Fig. 1 but for HR 4864

HD 111395 (HR 4864). Strassmeier et al. ([1997c]) presented some early photometric and spectroscopic data and concluded that the star is a spotted and chromospherically active, single, solar-type star of spectral type G5. They found a period of 16.95 days and sinusoidal light variations with an amplitude of around 0 $.\!\!^{\rm m}$02 in V. Part of their photometry is included in the data set in this paper and is from the Amadeus APT. Here, we basically confirm their results but refine the period to 15.80 days. Figure 26 shows a monotonic increase of the light curve amplitude from 0 $.\!\!^{\rm m}$02 at the beginning of the observing season up to 0 $.\!\!^{\rm m}$06 at the end. The phase curve in the right panel in Fig. 26 appears accordingly scattery.

Figure 27: U and V light curves for 31 Com (left two panels). The two middle panels show the spectral window (top panel) and the periodograms from the U-band data (three lower panels). The first plot is the original periodogram showing the one-day aliases (indicated by f1), the second plot is the periodogram with f1 subtracted. It shows the most likely period from our data set and is marked with f2 (0.1437 c/d = 6.96 days). The third plot is the periodogram with f2 subtracted. It identifies two frequencies (f3 and f4) that are both considered too weak though. The right panels are the phased U-light curves with the two frequencies f2 and, as a comparison, f3 (0.4331 c/d = 2.31 days), respectively. All data are from the Phoenix-10 APT

Figure 28: As in Fig. 1 but for IN Com. Top row: Amadeus $V(RI)_{\rm C}$ data. Bottom row: Wolfgang by data

HD 111812 (31 Com). This star is a rapidly rotating G0 giant that exhibits strong chromospheric activity. Consequently, cool starspots could be expected but, so far, no photometric variability due to rotational modulation was detected. Previous APT data was analyzed in Strassmeier et al. ([1997a]), and for more references we refer the reader to the discussion in that paper. No clear periodicity was seen but the star seemed to show seasonal brightness changes that were also noted in earlier observations by Lockwood et al. ([1997]). In this paper, we present further UBV data from the Phoenix-10 APT. Figure 27 shows the U and V light curves for 1996/97 (upper and lower left panels, respectively), the periodogram analysis for the U-band data (middle panels), and two phase curves with two possible periods (right panels). We verify the existence of seasonal brightness changes of approximately 0 $.\!\!^{\rm m}$01 in V. The period analysis of these data and their residuals revealed two possible periods besides the one-day alias (the alias is marked f₁ in Fig. 27): a 0.1437-c/d peak according to a period of 6.96 days (marked as f₂), and a 0.4331-c/d peak, i.e. 2.31 days, marked as f₃ (another frequency marked as f₄ is already considered to be too weak). Note that the phase curve with the 6.96-day period includes the systematic scatter due to the one-day aliasing and shows a full amplitude of no more than 0 $.\!\!^{\rm m}$01 in U. The Fourier amplitude of the 6.96-day period is twice as large as the 2.31-day period and we may consider it as the (preliminary) discovery of the rotation period of 31 Comae but emphasize that it is accordingly uncertain and possibly even spurious.

HD 112313 (IN Com). Photometry from various sources between 1983 and 1996 was collected and analysed by Strassmeier et al. ([1997a]), while Strassmeier et al. ([1997d]) obtained a Doppler image and a full season of additional photometry of this outstanding star. The star is the G5III-IV component of a triple system that includes one of the hottest known stars. The latter is the central star of the planetary nebulae LoTr-5. The rotation period of the G5 giant was unambiguously determined as 5.913 days (instead of 1.2 days) and made IN Comae a very rapidly rotating late-type star with an equatorial velocity of 95 kms^-1. As such, it very much resembles the class of (single) FK Comae-type stars. Our data for 1996/97 were gathered contemporaneously with both Vienna APTs to increase the time resolution and overcome the false period alarm. Both data sets showed the star with an amplitude of 0 $.\!\!^{\rm m}$06 in V and a period of 5.900 and 5.927 days for the Amadeus and Wolfgang data, respectively. Figure 28 shows the V and y light curves and the respective periodograms.

Figure 29: Seasonal V data and the periodogram for 37 Com. No single significant period was found

Figure 30: As in Fig. 1 but for FK Com. All data are from the Wolfgang APT and were taken in b and y

Figure 31: As in Fig. 1 but for HD 129333. Top row: Amadeus $V(RI)_{\rm C}$ data. Bottom row: Phoenix-10 UBV data

HD 112989 (37 Com). Strassmeier et al. ([1997b]) used 37 Com as the check star in their 1996 campaign on 31 Comae and found the star to be a long-term variable possibly with a period of around 80 days. 37 Comae was classified as a single G9IIICH-2 giant by Keenan & McNeil ([1989]). We obtained a single, high-resolution spectrum centered at 3950 Å at KPNO that shows weak Ca II H & K emission with asymmetric absorption reversals due to interstellar absorption, that indicates a giant luminosity class. $V\sin i$ from a red wavelength spectrum, obtained also at KPNO, is estimated to be 4 $\pm$ 2 kms^-1 (adopting a macroturbulence of 3 kms^-1). With a nominal radius of 10 $R_\odot$ for a G9III star, we could expect a rotation period in the range of approximately 70-170 days for $i\approx$ 45$^\circ$. Our data over a 150-day interval in Fig. 29 show no significant period above the S/N=4 criterion. A weak peak at 0.12 c/d (8.3 days) and its 1-f alias (1.1 days) is judged spurious. Despite of the increase of "scattered'' data toward the end of the observing season, we conclude that the star was constant in 1996/97.

HD 117555 (FK Com). Too numerous is the literature on FK Comae to be cited here and we refer the reader to the discussion of FK Comae in our previous APT paper (Strassmeier et al. [1997a]). Since then, Strassmeier et al. ([1997b]) presented four months of continuous and phase-resolved by data with the Wolfgang APT from early 1996 that were aimed to demonstrate the telescope's capabilities. In the present paper, we present the 1996/97 by data from Wolfgang. The y light curve in Fig. 30 shows a full 0 $.\!\!^{\rm m}$2 amplitude in late 1996 and early 1997 that smoothly decreased to 0 $.\!\!^{\rm m}$13 within 10 stellar rotations and then remained constant through the end of the observing season in late June 1997. The average photometric period was 2.4067 days, only 0.001% different from the period from early 1996.

Figure 32: As in Fig. 1 but for UV CrB. Top row: Amadeus $V(RI)_{\rm C}$ data. Bottom row: Wolfgang by data

Figure 33: As in Fig. 1 but for UZ Lib

Figure 34: Strömgren b light curve for $\gamma$ CrB minus $\alpha$ CrB

Figure 35: $\delta$ CrB minus $\alpha$ CrB. Two (primary) eclipses are seen due to $\alpha$ CrB and a long-term 28-day variation due to $\delta$ CrB

Figure 36: As in Fig. 1 but for HD 152178

Figure 37: As in Fig. 1 but for HD 171488. The upper three panels are data from Amadeus and show Johnson V, the lower panels are from Wolfgang and show Strömgren y

Figure 38: As in Fig. 1 but for HD 199178

Figure 39: As in Fig. 1 but for HD 208472

Figure 40: As in Fig. 1 but for HK Lac

Figure 41: As in Fig. 1 but for HR 9024

Figure 42: As in Fig. 1 but for IM Peg

HD 129333 (EK Dra). The present $BV(RI)_{\rm C}$ data on EK Dra were partially analysed in Strassmeier & Rice ([1998a]) where they were used as additional constraint for Doppler imaging. Here, we simply present the numerical data from the Amadeus and Phoenix APTs from 1996/97 and perform a period analysis of the separate data sets. The best periods were 2.597 days and 2.601 days for the Amadeus and Phoenix-10 data, respectively. Thus, we confirm our earlier photometric period of 2.599 days from the combined V data. For reasons of completeness, we plot the data in Fig. 31 along with a periodogram from the V data.

HD 136901 (UV CrB). This star is a massive ellipsoidal K1 giant in a single-lined spectroscopic binary (Fekel et al. [1989]). Its light variations are attributed mainly to the ellipticity effect with some contribution from the reflection effect (Strassmeier et al. [1989]). Recently, Fekel et al. ([1999]) presented a new orbit and summarized the star's fundamental properties. Kaye et al. ([1995]) suceeded in extracting the spot component out of the combined light curves and found amplitudes between 0 $.\!\!^{\rm m}$048 and 0 $.\!\!^{\rm m}$009 in V. For 1996/97, we had UV CrB on both Vienna APTs in order to search for further evidence of starspot activity. Figure 32 shows the data from both telescopes along with the periodograms and phased light curves. The best-fit frequency was 0.107 c/d (9.33 days) but, because the ellipticity effect causes a double-humped light curve, the true period is at f/2. Thus, the light curve in Fig. 32 was phased with a period of 18.657 days. The scatter in the seasonal phase plot from the Amadeus telescope is mostly due to intrinsic changes during the observing season. Note that the time coverage of the Wolfgang data is only one half that of the Amadeus data.

BD-08$^\circ$3999 (UZ Lib). Recently, Fekel et al. ([1999]) presented a new orbit determination and summarized the star's fundamental properties. Strassmeier et al. ([1997a]) obtained a Doppler image of UZ Lib and we refer to their discussion for further relevant literature on this star. The 1996/97 season's photometry shows the star with a relatively small V amplitude of 0 $.\!\!^{\rm m}$1 and a doubled-humped light-curve shape but confirms the 4.7-day period. Note that in this case the periodogram from sine-curve fitting gives smaller residuals at one half the true period. Figure 33 shows the light curves and the periodogram.

HD 139006 ($\alpha$ CrB). $\alpha$ CrB is a known eclipsing binary with a bright and dominating A0V primary and a comparably small G5V secondary with an orbital period of 17.4 days (Tomkin & Popper [1986]). At X-ray wavelengths, the A star is not detected at all and Schmitt & Kürster ([1993]) used this fact to presented an image of the corona of the G5 secondary from X-ray eclipse mapping. Recently, Schmitt ([1998]) reported the discovery of apsidal motion in $\alpha$ CrB from X-ray eclipse timings. Our photometry during times outside eclipses indicates no variations above the nominal noise level. Two primary eclipses were covered and showed a depth of only 0 $.\!\!^{\rm m}$03 in y. See Fig. 35 for an (inverse) light curve of the $\alpha$ CrB eclipses.

HD 140436 ($\gamma$ CrB). This star was the comparison star in the $\alpha$ CrB group and turned out to be the suspected "Maia''-type variable $\gamma$ CrB (Lehmann et al. [1997]). The history of light variability for this bright (V = 3 $.\!\!^{\rm m}$8) star is checkered. Originally, the variability was discovered by Fernie ([1969]) and confirmed by Percy ([1970]) from a more extensive dataset that showed an amplitude of up to 0 $.\!\!^{\rm m}$05 in V and B and a period of 0.03 days. Veto & Kovacs ([1981]) monitored $\gamma$ CrB during four nights in 1981 but found no variability above 2-5 millimag. The recent "Catalog of variable stars in the lower instability strip'' of Garcia et al. ([1995]) lists $\gamma$ CrB as a variable with the parameters found by Percy ([1970]). In the course of the preparation of this paper, we learned that photometric variations were also detected by Scholz et al. ([1998]).

Our frequency analysis of the 1996/97 Wolfgang data shows the strongest reduction of the sum of the squared residuals at a period of 0.44534 days and with an amplitude of 0 $.\!\!^{\rm m}$010 in b and 0 $.\!\!^{\rm m}$005 in y. Note that the period from the y data (0.44543 days) is less significant due to the much smaller amplitude but is consistent with the b period. The periodogram from the b data in Fig. 34 shows a series of additional peaks above the S/N=4 threshold but they are all aliases of the 0.445-day period ( f₀=2.24548), including the $f_0\pm1$ periods of 0.8031 days and 0.3081 days. In a recent series of papers, Lehmann et al. ([1997]) detected radial-velocity variations of $\gamma$ CrB with a period of 0.44499 days and concluded that twice this period, i.e. 0.89 days, is the fundamental period. They suggested that this must be the rotation period of $\gamma$ CrB because a 0.445-day period would indicate a (less likely) very low inclination of the stellar rotation axis of $i\approx$ 20$^\circ$ and consequently the very high equatorial rotational velocity of 270 kms^-1. Two more periods were cited by Lehmann et al. ([1997]), i.e. 0.1271 days and 0.0989 days, but neither of them shows up in our photometric data. Because the 0.89-day period (i.e., f₀/2) is much weaker than the 0.445-day period in our data set, we conclude at the moment that the 0.44534-day period is the true period of $\gamma$ CrB. The spectral classifications listed in SIMBAD are B9IV, A0IV, or A0V, which suggests that its light variability is most likely due to a combination of rapid rotation and non-radial pulsations, but that remains to be determined.

HD 141714 ($\delta$ CrB). $\delta$ CrB was chosen as the check star for the $\alpha$ CrB group and is a known chromospherically active, single, G5 giant (Baliunas [1987]). Choi et al. ([1995]) determined its rotation period from Mt Wilson H&K-survey data as well as from broad-band photometry to 59 days, and thus confirmed the 60.8-day period found earlier by Fernie ([1991]). Unfortunately, our comparison star turned out to be a short-period variable ($\gamma$ CrB, see above) and its variations are modulated into the $\delta$ CrB photometry. However, our actual target star, $\alpha$ CrB, is constant outside of eclipse and we use it as the comparison star for all differential magnitudes of $\delta$ CrB. Our period analysis of these data does not show the strongest peak at the anticipated 60-day period but at 28.5 days (f=0.0351). Note that even visually the y-band light curve in Fig. 35 indicates this period and not a 60-day period. However, if the light curve appeared double-humped during the times of our observations, we would measure only one half of the true period. Therefore, we consider our true period to be $2\times P_{\rm phtm}=57$ days. Due to the small amplitude of just 0 $.\!\!^{\rm m}$012 in y, this period is uncertain by $\approx$4 days and is therefore still within the range of previous period determinations. Figure 35 shows our light curve and periodogram from 1996/97.

HD 152178 (V2253 Oph). Houk ([1982]) had noted the Ca II H & K emission of this SB1 binary. This prompted Strassmeier et al. ([1993]) to include it in the second edition of the catalog of chromospherically active binary stars. Later, the Ca II H & K emission was confirmed from high-resolution spectra by Strassmeier et al. ([1994b]). The light variability with a period of 22.35 days was discovered by Hooten & Hall ([1990]) while Eaton et al. ([1996]) presented a V light curve from the full observing season 1989 to demonstrate the rapid decrease of its amplitude from 0 $.\!\!^{\rm m}$2 to 0 $.\!\!^{\rm m}$05 within 100 days. Fekel ([1997]) revised the projected rotational velocity from 24 to 28.8 kms^-1 and, recently, Fekel et al. ([1999]) presented an orbit determination with an orbital period of 314.47 days. Our new data from 1996/97 were gathered with the Amadeus APT and showed a maximum amplitude of 0 $.\!\!^{\rm m}$15 in V. The periodogram in Fig. 36 is dominated by the many aliases of the one-day observing period but show the 22-day period as a significant frequency at 0.0453 c/d (22.07 days).

HD 171488 (V889 Her). Henry et al. ([1995b]) and Cutispoto et al. ([1998]) presented photometry for this G0V star and both groups independently found its light variations with an amplitude of 0 $.\!\!^{\rm m}$1 and a period of 1.338 days. Such a short period is also in agreement with the dwarf classification and the measured rotational velocity of 33 kms^-1 (Henry et al. [1995b]). For further references on this star, we refer the reader to the discussion in Henry et al. ([1995b]). Our 1996/97 season photometry was obtained with both Vienna APTs and showed HD 171488 with a quite variable light amplitude between 0 $.\!\!^{\rm m}$1 in mid March and 0 $.\!\!^{\rm m}$01 just 20 days later, and the same again in mid May (see Fig. 37). The seasonal average photometric period with the largest reduction of the $\chi^2$ is 1.340 days (f=0.746 c/d) in the Amadeus data set and 0 $.\!\!^{\rm d}$792 days (f=1.263 c/d) in the Wolfgang data set. A frequency of 0.740 c/d is present in the Wolfgang data as well, as does the 1.263-c/d frequency in the Amadeus data, but both appear comparably weaker (see Fig. 37). The combined V+y data show a 0.748-c/d frequency (1.337 days) as the highest peak (the y magnitudes were transformed to V using the relation in Olsen ([1983]). Therefore, we confirm the existence of the 1.34-day period but emphasize that there is a strong alias of 0.79-days in our data sets. HD 199178 (V1794 Cyg). HD 199178 belongs to the FK Comae group of rapidly rotating single giants and has been the target of numerous investigations (see, e.g., Strassmeier et al. [1999a]; Jetsu et al. [1993]; Dempsey et al. [1992] and references therein). Most recently, Jetsu et al. ([1999]) reinvestigated all available UBV photometry up to 1996 and found unpredictable activity shifts that resulted in significantly different photometric periods from year to year and even within one year. These periods are between 3.81 and 3.14 days with an average of 3.3175 days. Our data from the observing season 1996/97 indicate a period of 3.342 days and an average amplitude of 0 $.\!\!^{\rm m}$12 in V. Note that our comparison-star magnitudes (SAO50313) were also transformed to Cousins $V(I)_{\rm C}$, and we found V = 6 $.\!\!^{\rm m}$649 $\pm$ 0 $.\!\!^{\rm m}$013 and $I_{\rm C}$ = 5 $.\!\!^{\rm m}$592 $\pm$ 0 $.\!\!^{\rm m}$016 for SAO50313.

Figure 43: Seasonal V light curve for the S-giant HR Peg

Figure 44: As in Fig. 1 but for HD 218153

Figure 45: As in Fig. 1 but for II Peg

HD 208472 (V2075 Cyg). Henry et al. ([1995b]) discovered the light variability of this G8III giant and also found it to be a single-lined spectroscopic binary with an orbital period of 22.6 days. Recently, Fekel et al. ([1999]) presented an orbit determination and summarized the star's fundamental properties. The photometric period of 22.54 days suggests a synchronous rotator despite that the eccentricity of the orbit was not explicitely determined but must be very close to zero. Recently, Fekel ([1997]) revised his earlier $v\sin i$ determination from 21 to 19.7 kms^-1. Our new photometry was gathered with the Amadeus APT. While the amplitude was 0 $.\!\!^{\rm m}$12 in November 1996, it decreased to 0 $.\!\!^{\rm m}$07 in June 1997. Note that Henry et al. ([1995b]) had seen the star with an amplitude of 0 $.\!\!^{\rm m}$36 in V in 1993/94. Such large changes are typical for only the most active stars. The best-fit period from the Amadeus data was 22.42 $\pm$ 0.12 days.

HD 209813 (HK Lac). Oláh et al. ([1997]) presented and analysed 30 years of photoelectric observations of HK Lac and we refer the reader to the many references in that paper. Our new data from the 1996/97 observing season are from the Amadeus APT and showed a full 0 $.\!\!^{\rm m}$1 V amplitude in November/December 1996 which increased to 0 $.\!\!^{\rm m}$15 by June 1997. A shallow secondary minimum was seen at the beginning of the season (Fig. 40) but had vanished by June. The photometric period was 24.15 days in agreement with previous determinations and the orbital period.

HR 9024 (OU And). Hopkins et al. ([1985]) discovered the light variability of HR 9024 and determined a preliminary period of between 22-24 days. Cowley & Bidelman ([1979]) classified the spectrum as G1III and discovered the Ca II H & K emission. Several years of photometry were presented by Strassmeier & Hall ([1988b]) who found a photometric period of 22.6 days. Fekel et al. ([1986]) and Strassmeier et al. ([1989]) determined further relevant stellar parameters. HR 9024 was detected in the ROSAT all-sky survey by Huensch et al. ([1998]) which proved that HR 9024 is also coronally active. The photometric amplitude in 1996/97 amounted to barely 0 $.\!\!^{\rm m}$01 in V but revealed a declining trend towards the end of our observations in June 1997. The period from our data set is 24.2 days but is not well defined and accordingly uncertain. We note that the minimum radius of 9.6 $R_\odot$ from $v\sin i$ and the photometric period (Table 1) is in severe disagreement with the spectral classification of G1III. Unless the light curve of HR 9024 is double humped, and thus the true period twice as long as the one reported in this and previous papers, we recommend further monitoring of this bright star. Figure 41 shows the seasonal V light curve, the periodogram, and the phased V light curve.

HD 216489 (IM Peg). Previous APT data were analyzed in Strassmeier et al. ([1997a]) while Fekel et al. ([1999]) presented an orbit determination and also summarized the star's fundamental properties. For more details on IM Peg, we refer the reader to the discussions in these papers. The 1996/97 photometry showed the star with a large 0 $.\!\!^{\rm m}$4 V amplitude in November/December 1996 which declined to approximately 0 $.\!\!^{\rm m}$22 by June 1997. The photometric period was 24.45 days. Figure 42 shows the light curves and the periodogram from the V data.

HD 216672 (HR Peg). This was our check star for the Amadeus observations of IM Peg and is the semi-regular S-type giant HR Pegasii (= HR 8714, e.g. Eggen [1992]) and, since our comparison star was proven to be constant, we present a seasonal light curve for HR Peg in Fig. 43.

HD 218153 (KU Peg). Weber et al. ([1999]) presented three Doppler images obtained from data from three consecutive rotations of the star in late 1996. Parts of our APT data were already used by them as additional input for the line profile inversion. Here, we present the complete dataset for the entire observing season 1996/97 and obtain a photometric period of 25.9 days.

HD 224085 (II Peg). Berdyugina et al. ([1998a],[1998b]) presented Doppler images of II Peg and also updated the orbital and atmospheric parameters of this very active RS CVn binary and we refer the reader to that paper and the literature cited therein. Our new photometric data from 1996/97 show a period of 6.725 days, very close to the orbital period of 6.72433 days

Acknowledgements

Research with the APTs at the University of Vienna is supported by the Austrian Fond zur Förderung der wissenschaftlichen Forschung (FWF) through grant S7301-AST. Special thanks go to Mike Seeds of Franklin & Marshall College for keeping the Phoenix-10 APT busy and to the whole Tennessee State University APT group for continuous collaboration. It is also a great pleasure to thank Lou Boyd and Don Epand of Fairborn Observatory for their continuous operation of and help with the Wolfgang-Amadeus APTs and their data reduction. The careful reading and suggestions of the referee, Dr. J. Bouvier, are also appreciated. This research has greatly benefited from use of the SIMBAD data base, operated at CDS in Strasbourg, France.