3. Results

All of our program stars except 31 Comae are known variable stars and were chosen on the grounds that they are part of an ongoing campaign on Doppler imaging or another spectroscopic project, or because there was simply a lack of photometry. Table 1 (click here) lists the program stars and summarizes their most relevant properties taken from the second edition of the Catalog of Chromospherically Active Binary Stars (Strassmeier et al. 1993a) or, if just recently discovered or in case of a single star, from literature quoted in Sect. 4 (click here) in this paper.

3.1. Photometric periods

Table 4 (click here) summarizes the results of our period analysis. We used a menu-driven Fortran program that performs a multiple frequency search through minimization of the residuals with a Fourier option (Kollath 1990, but see also Breger 1990). Errors are estimated from the width of the frequency peak at where n is the number of data points and m the number of parameters (n-m is then the number of the degrees of freedom; m=4 in our case). For details we refer to Bevington (1969). Figure 1 (click here) shows the periodogram for the combined and seasonal V data of HD 12545 and FK Com and the respective window functions from the various observing modes over the years. This figure is representative for the periodogram analysis in this paper. All photometric periods in Table 4 (click here) were obtained from the V data only, mostly because of the particular good time coverage in V and included our own as well as many of the previously published data listed in Table 5 (click here). Some of the data have been prewhitened and if so this is indicated in Sect. 4 (click here). No systematic zero point adjustments were necessary.

  
Figure 1: Three examples of the periodogram analysis in this paper. Shown are the window func tions for the respective observing epoch (lower panels) and the periodograms as amplitude versus frequency in cycles per day in the upper panels. The two left panels are for the combined V data of HD 12545 from 1985 to 1996 and for one particular observing season (1995/96), respectively while the right panel is the periodogram for 30 years of V-band photometry of the rapidly-rotating single giant FK Comae. The error of a period is determined from the half width of the minimum at the level where the residuals are 1/n-times larger than at the best-fit period (n being the total number of datapoints)

The periods designated in Table 4 (click here) are the long-term average photometric periods computed from the full data sets including the literature data and probably best represent the stellar rotation period. All light curves in this paper are phased with this period unless otherwise noted. The corresponding zero point in time, T₀, is the arbitrary time of the first observation in each data set. Deviations of the seasonal periods from this long-term average can be directly associated with differential surface rotation in case the seasonal data coverage permits a sufficiently precise period determination.

   

91-92 92-93 93-94 94-95 95-96 (2 400 000+) (days) Time range

HD 12545 25.32 24.08 24.69 24.01 23.87 46370.0 24.0801 1985-96

HD 17433 16.515 17.29 16.242 16.308 16.318 40029.0 16.1996 1973-96

EI Eri 1.9352 1.9419 1.9444 1.9436 1.9456 44129.0 1.952717 1980-96

V410 Tau 1.87182 1.87159 ... 1.87089 1.87299 37962.0 1.872025 1962-96 ...

V833 Tau 1.8522 1.7737 1.8015 1.7963 1.8982 45618.0 1.791601 1984-96

SU Aur ... ... 2.742 ... ... 41952.0 2.69935 1973-94

... ... ... ...

HD 31738¹ ... ... ... ... ... ... ... 1979-96

HD 31993 ... ... 27.78 28.09 27.62 43856.0 28.1246 1979-96

... ...

HD 39576 2.796 2.286 2.2655 ... 2.2806 48636.0 2.26883 1992-96 ...

HD 81410 .... 12.669 12.723 12.864 12.826 40988.0 12.78642 1971-96 ...

HD 82443 ... ... 5.492 5.413 5.409 47567.0 5.40482 1989-96

... ...

LQ Hya 1.60769 1.6154 1.60555 1.6033 1.60689 45275.0 1.600881 1984-96

HD 106225 10.353 10.424 10.102 10.327 10.448 45110.0 10.4181 1982-96

31 Com¹ ... ... ... ... ... ... ... 1984-96

IN Com ... 5.903 5.903 5.958 5.9108 45444.0 5.93474 1983-96 ...

HD 113816 ... 24.05 23.17 23.51 24.50 49025.0 23.674 1993-96 ...

IN Vir ... ... 8.281 8.155 8.1500 47949.0 8.18833 1990-96

... ...

FK Com 2.4064 2.4035 2.40211 2.3896 ... 39253.0 2.397696 1966-95 ...

HD 129333 ... ... 2.796 2.6023 2.6033 49403.0 2.60498 1994-96

... ...

UZ Lib ... 4.7655 4.7343 4.7659 4.654 41396.0 4.752264 1972-96 ...

HD 195040² ... 22.32 11.569 11.542 11.64 44355.47 11.5937 1979-96 ...

HD 202077 ... 14.67 14.65 15.37 14.69 44071.0 14.7957 1979-96 ...

IM Peg ... ... 25.19 24.108 24.498 43734.0 24.4936 1978-96

... ...

¹No short-term light variability detected. See Discussion.
²Double-humped light curve. Seasonal data are phased with the orbital period of 23.206 days.

For those binaries in our sample where the rotation is sufficiently synchronized to the orbital motion and only few data were available we chose the orbital period instead of the long-term photometric period to phase the data. These stars are marked in Table 4 (click here). An orbital period is usually known with higher precision and accuracy than a photometric period and is thus a good timekeeper. However, there is some theoretical support that the convective motion in the outer layers of close late-type binary components is governed by the orbital angular momentum (Schrijver & Zwaan 1991) and care must be exercised when data from long intervals in time are phased together. Most eclipsing RS CVn binaries show indeed significant orbital-period variations. The prototype RS CVn, for example, exhibits variations of up to 0.2 days in about 40 years (Rodonó et al. 1995).

3.2. Long-term brightness variations

It is now a well-established fact that solar-type stars exhibit the same long-term cycles in their photospheric output as originally discovered from the chromospheric CaII H&K emission (e.g. Baliunas & Soon 1995; Gray et al. 1996; 1992), thus strengthening the solar-stellar connection. For evolved stars, however, a study of variability in a sample of G and K giants by Choi et al. (1995) showed only one clear detection ( CrB) of a rotationally modulated signal from both V-band photometry and CaII-S-index spectrophotometry. It is thus not surprising that long-term changes in the mean brightness of RS CVn-type stars are not a straightforward tool to detect spot cycles similar to the Sun's 11-year cycle despite the now rapidly growing data base. There are basically three reasons for this: first, very active evolved and, compared to the Sun, heavily spotted stars might not have such strictly cyclic behavior and/or, second, their long-term cycles are of relatively small amplitude and are masked by short-term irregularities such as dynamo-unrelated redistributions of the starspot asymmetry (note that only the asymmetric part contributes to the light-curve amplitude) and/or, third, their cycle periods are much longer than those found for solar-type stars (7 to 21 years according to Baliunas & Soon 1995).

Convincing observational evidence for the existence of long-term and cyclic brightness variations in very-active late-type dwarf stars comes from the photographic magnitudes measured off archival plate collections. Hartmann et al. (1981) discovered a 60-year cycle with an amplitude of 0.5 mag for V833 Tau (K5V) from plates dating back to the early 1900's. A similar periodicity with an amplitude of 0.3 mag was found for BY Dra, the prototype BY-Draconis system consisting of two late K-dwarf stars (Phillips & Hartmann 1978). Just recently, Bondar (1995) discovered an even larger amplitude (0.8-1.0 mag) long-term variable: PZ Mon with a cycle period of 50 years. Again, PZ Mon is a dK2e-type main-sequence flare star similar to V833 Tauri. But what about evolved stars?

A baseline of 15-19 years of photometry of the four RS CVn binaries  And,  Gem, II Peg and V711 Tau was collected and analysed by Henry et al. (1995b). Long-term variability with a periodic behavior in mean brightness was discovered in all four of them: 11.1 yr for  And, 8.5 yr for  Gem, 11 yr for II Peg, and 16 yr for V711 Tau. We may note, however, that these periodicities are not so obviously apparent from a plot of the long-term lightcurves because of the drastly variable rotational modulation in these stars but the existence of long-term variations seems well established. An even longer baseline in time (45 years) is available for RS CVn itself, a K2 subgiant in a 4.8-day orbit. Rodonó et al. (1995) found that the total area covered by spots shows a cyclic variation with a period of 20 years. Obviously, cycle periods in active RS CVn-type stars can be of the same order than those in solar-type stars but tend to be, on average, somewhat longer. Therefore, one of our goals in this paper is to built up a sufficiently long data base that enables the detection of such variations - periodic or not.

Figures 2 (click here) through 24 present all currently available V-band photometry for our program stars. Supplemental data are taken from the literature and are summarized in Table 5 (click here). Literature data that were available in numerical form are plotted as individual points, normally representing the average of three individual readings, while published light curves without numerical data are entered as vertical bars indicating their maximum and minimum brightness. Note that Fig. 25 (click here) shows the long-term light and color curves for the check star of IM Peg (=HR Peg).

   

Star Epoch Reference

HD 12545 46370-47947 Skiff & Nations (1991)

46654-46882 Hooten & Hall (1990)

48148-48323 Nolthenius (1991)

48502-48691 Strassmeier & Oláh (1992)

48171-49708 Hampton et al. (1996)

HD 17433 42029-42344 Chugainov (1976)

45948-45973 Eaton & Poe (1986)

46014-46761 Bopp et al. (1989)

47065-47213 Strassmeier & Bopp (1992)

1992.7,1993.7 Alekseev (1996)

EI Eri 44129-44974 Lloyd-Evans & Koen (1997)

44287-44660 Fekel et al. (1982)

44520-44660 Bopp et al. (1983)

45260-45350 Hall et al. (1987)

45304-45400 Baliunas et al. (1983)

45700-46483 Strassmeier et al. (1989a)

46821-47232 Strassmeier (1990)

47129-47143 Hooten et al. (1989)

47112-47602 Hall (1996)

47480-47511 Barksdale (1990)

1988.0-90.0 Rodonó & Cutispoto (1992)

47869-47888 Cutispoto (1995)

V410 Tau 37962-41734 Romano (1975)

42469-44636 Rössiger (1981)

44173-44176 Herbig & Goodrich (1986)

44227-44659 Rydgren et al. (1984)

44897-44904 Rydgren & Vrba (1983)

45641-46862 Vrba et al. (1988)

46012-46150 Holtzman et al. (1986)

46044-46796 Bouvier et al. (1988)

46432-47940 Herbst et al. (1994)

47082-47612 Herbst (1989)

1993 Grankin (1994)

46660-48935 Petrov et al. (1994)

V833 Tau 37699-45674 Eggen (1984)

44136-44524 Hartmann et al. (1981)

45618-47929 Oláh & Pettersen (1991)

48151-49066 Oláh & Jurcsik (1996)

SU Aur 41953-42006 Rydgren et al. (1976)

42357-42363 Cohen & Schwartz (1976)

39095-44637 Rydgren et al. (1984)

44860-45399 Herbst et al. (1983)

45282-45313 Gahm et al. (1993)

45979-46022 Holtzman et al. (1986)

46044-46796 Bouvier et al. (1988)

46313-47605 Herbst et al. (1994)

46313-46493 Herbst et al. (1987)

47162-47174 Herbst & Koret (1988)

48212-48272 Bouvier et al. (1993)

HD 31738 43856-44967 Lloyd-Evans & Koen (1987)

44600-45400 Hall et al. (1986)

45971-46505 Strassmeier et al. (1989a)

47121-47434 Hall (1988)

47869-47887 Cutispoto (1995)

HD 31993 43856-44967 Lloyd-Evans & Koen (1987)

47417-47434 Hooten & Hall (1990)

HD 39576 48636-48644 Strassmeier et al. (1992)

 

Star Epoch Reference

HD 81410 40988-41503 Eggen (1973)

43860-45359 Lloyd-Evans & Koen (1987)

44363-44738 Collier-Cameron (1987)

44608-44678 Raveendran et al. (1982)

1988 Mekkaden & Sinachop. (1988)

46066-49390 Manfroid et al. (1991)

47129-47143 Pallavicini et al. (1993)

47148-48077 Sterken et al. (1993)

47548-47595 Cutispoto (1993)

47869-47888 Cutispoto (1995)

48189-48658 Manfroid et al. (1995)

48957-49390 Sterken et al. (1995)

HD 82443 47567-47674 Messina & Guinan (1996)

49638-49717 Henry et al. (1995a)

LQ Hya 45275-45779 Fekel et al. (1986)

45699-45734 Eggen (1984)

46023-46529 Strassmeier & Hall (1988b)

47133-47143 Cutispoto (1991)

47548-47593 Cutispoto (1993)

47881-48760 Jetsu (1993)

47949-47961 Cutispoto (1996)

48270-48300 Strassmeier et al. (1993b)

HD 106225 45110-45153 Fekel et al. (1984)

46832-46841 Cutispoto (1990)

47548-47594 Cutispoto (1993)

47949-47961 Cutispoto (1996)

47987-48437 Hall & Henry (1992)

48270-48433 Strassmeier (1994)

31 Com 45722-46603 Strassmeier & Hall (1988a)

47163-47232 Hall (1988)

49372-49521 Jasniewicz et al. (1996)

IN Com 45444-45525 Schnell & Purgathofer (1983)

45674-47288 Schnell (1996)

1985,1986-89 Malasan et al. (1991)

46883-47305 Noskova (1989)

1988.4,1989.7 Bond & Livio (1990)

47604-47691 Jasniewicz et al. (1994)

48979-49120 Jurcsik (1996)

49100-49107 Kuszawska & Mikolaj. (1993)

49372-49521 Jasniewicz et al. (1996)

HD 113816 49415-49528 Henry et al. (1995a)

IN Vir 47949-47961 Cutispoto et al. (1996)

FK Com 39253-39346 Chugainov (1966)

42126-42223 Chugainov (1976)

42829-42837 Rucinski (1981a)

43624-43664 Olsen (1980)

44651-44661 Morris & Milone (1983)

45032-45207 Dorren et al. (1984)

45045-48101 Jetsu et al. (1993)

45116-45139 Bohusz & Udalski (1983)

45376-45388 Holtzman & Nations (1984)

45351-45501 Cellino et al. (1986)

45466-45861 Huovelin et al. (1987)

45796-45802 Oja (1985)

45790-45803 Eaton (1985)

47292-47303 Heckert & Maloney (1992)

 

Star Epoch Reference

FK Com 1990.4 Marchev & Kjurkchieva (1993)

48025-48403 Heckert et al. (1992)

48754-48838 Heckert (1993a)

49034-49120 Oláh & Jurcsik (1996)

49804-49833 Petreshock et al. (1995)

HD 129333 46567-46588 Dorren & Guinan (1994)

1991.4 Chugainov et al. (1991)

49377-49410 Scheible & Guinan (1994)

UZ Lib 1930 Parenago (1931)

41396-41501 Wisniewski (1973)

1977.3 Hoffmann (1980)

43944-43957 Rucinski (1981b)

45423-45485 Bopp et al. (1984)

1983.2 Nations & Holtzman (1987)

46166-46628 Grewing et al. (1989)

1988.4,1989.5 Heckert & Hickman (1991)

1990.4,1991.3 Heckert (1992)

48754-48844 Heckert (1993b)

49556-49564 Paunzen & Strassmeier (1996)

HD 195040 43785-45166 Lloyd-Evans & Koen (1987)

44088-44161 Collier-Cameron (1987)

44461-44485 Fekel et al. (1986)

HD 202077 44071-44912 Lloyd-Evans & Koen (1987)

49528-49549 Strassmeier et al. (1994b)

IM Peg 40876-41580 Herbst (1973)

43734-44576 Eaton et al. (1983)

44049-44212 Percy & Welch (1982)

45620-46601 Strassmeier et al. (1989a)

45957-45973 Eaton (1985)

48502-48654 Oláh (1996)

48859-48979 Dempsey et al. (1996)

3.3. Seasonal light curves

Figures 26 (click here) through 48 present the photometric data for the stars in Table 1 (click here). All of the individual figures are grouped into observational seasons such that one V-band plot versus Julian date (JD) appears on the upper left side and a phase plot of the entire season's data on the upper right side of each graph. The lower panels show the B-V and/or the colors, whenever observed. Individual symbols are identified in the respective panels. The season is indicated at the top of each panel. Furthermore, data from the different APTs as well as data taken from the literature are identified with separate symbols as indicated. We emphasize that the sometimes large scatter in the phase plots is almost exclusively due to intrinsic light curve changes and not due to instrumental scatter.