The present multicolor photometry has been used to investigate the light
curve evolutions with respect to previous observations, to search for
the presence of photospheric solar-like activity cycles and, using
the method described
by Cutispoto et al. (1996),
to infer or further constrain
the spectral type and luminosity class of the program stars (see Table 3 (click here)).
The results for the individual stars are discussed below.
Figure 4: V-band light curve and colors of HD 16157 = CC
Eri. Phases are reckoned from the ephemeris 
, where the orbital period is from Evans (1959)
Table 3: Maximum luminosity (
) and corresponding colors for
the program stars; standard deviations (
) for the v-c and ck-c
V-band differential magnitudes for each series of N nights; inferred
spectral classification (Spectral Type)
Figure 5: V light long-term variability of HD 16157 = CC
Eri. The vertical bars indicate the peak-to-peak amplitude of the light curve
Figure 6: V-band light curve and colors of
HD 32918 = YY Men. Phases are reckoned from the photometric ephemeris
(Collier 1982)
Figure 7: V light long-term variability of HD 32918 = YY
Men. The vertical bars indicate the peak-to-peak amplitude of the light curve
Figure 8: V-band light curve and colors of
HD 155555 = V824 Ara. Phases are reckoned from the spectroscopic ephemeris
(Pasquini et al. 1991)
Figure 9: V light long-term variability of HD 155555 =
V824 Ara. The vertical bars indicate the peak-to-peak amplitude of the light
curve
Figure 10: V-band light curve and colors of
HD 174429 = PZ Tel. Phases are reckoned from the ephemeris HJD = 2448141.5 +
0.94486 
E, where the photometric period is from Innis et al. (1990)
Figure 11: V light long-term variability of HD 174429 = PZ
Tel. The vertical bars indicate the peak-to-peak amplitude of the light curve
HD 5303 = CF Tuc is a bright RS CVn-type eclipsing system
that has been monitored by several authors in recent years (cf. Strassmeier
et al. 1993; Cutispoto 1995; Budding & Zeilik 1995
and references therein).
It was detected during the ROSAT and EUVE all-sky EUV surveys (Pounds et al.
1993; Pye et al. 1995; Malina et al. 1994).
The observations carried out with
the 50 cm and the 1.0 m over the intervals 7-14 Sep. and 1-9 Oct.,
respectively are shown in Fig. 1 (click here), where phases are reckoned
from the spectroscopic ephemeris 
(Budding 1985). The non-flat
out-of-eclipse V-band light curve is indicative of the presence
of photosperic spottedness and, with respect to previous observations
(Cutispoto 1995; Budding & Zeilik 1995 and
references therein), displays a different luminosity maximum and
shape. The remarkable light curve modifications observed for CF
Tuc since late 1978 have been interpreted by Budding &
Zeilik (1995) in terms of a spot cycle lasting 5-6 years. The
spectral classification of CF Tuc is not well established (see
Cutispoto 1995 and references therein). The 
values reported by Randich et al. (1993) lead to
stellar radii of about 
and 
for the
hotter and cooler components, respectively while Coates et al.
(1983) computed 
and 
,
respectively. These values indicate the presence of two evolved stars,
both of luminosity class IV or V/IV. In fact, the best match of the
observed colors is obtained by assuming the system to be formed by
G0 V/IV + K4 V/IV or by G5 IV + K4 V/IV components, with the former
solution in better agreement with the computed radii.
Figure 12: V-band light curve and colors of
HD 197481 = AU Mic. Phases are reckoned from the photometric ephemeris
(Torres et al. 1972)
Figure 13: V-band light curve and colors of
HD 214479 = FK Aqr. Phases are reckoned from the photometric ephemeris
Figure 14: V-band light curve and colors of
HD 217344 = TZ PsA. Phases are reckoned from the ephemeris 
, where the photometric period is from Lloyd Evans & Koen
(1987)
Figure 15: V light long-term variability of HD 217344 = TZ
PsA. The vertical bars indicate the peak-to-peak amplitude of the light curve
HD 8435 = BC Phe is a SB2 spectroscopic binary with an
orbital period of 0.657 days (Balona 1987). Photometric observations have been
obtained by Lloyd Evans & Koen (1987); Collier Cameron (1987)
and Cutispoto
(1995). Low Li abundance is reported by Pallavicini et al. (1992).
No microwave
emission was detected by Slee et al. (1987).
The observations obtained with the
0.5 m over the interval 7-14 Sep. are shown in Fig. 2 (click here).
The light curve has been
folded by using the 0.649-day photometric period inferred by Cutispoto (1995).
Comparing the data in Fig. 2 (click here)
with those obtained about 9 months earlier by
Cutispoto (1995), a remarkable evolution of the light curve is evident.
In fact, the present data show two clear and almost identical maxima, while the
global degree of spottedness has decreased, as can be inferred from the
collection of the available photometry presented in Fig. 3 (click here).
The colors are
consistent with the G7/8 V/IV + K3/5 V spectral classification suggested by
Cutispoto (1995).
HD 16157 = CC Eri is a BY Dra-type variable that has been
studied at different wavelengths by several authors (see Byrne et al. 1992;
Cutispoto 1991 and references therein). More recently, it was detected by
the ROSAT and EUVE satellites during their all-sky surveys (Pye et al. 1995;
Malina et al. 1994; Pounds et al. 1993). The observations obtained with the
0.5 m
over the interval 7-14 Sep. are shown in Fig. 4 (click here),
where phases are reckoned from
the 1.56145-day orbital period (Evans 1959).
Clear color variations, partially
correlated with the V-band modulation, are present for all the color
indices. Comparing the present data with recent photometric observations
(Cutispoto 1991; 1992; 1995)
a remarkable decrease in the maximum luminosity
emerges (cf. Fig. 5 (click here)).
In fact, the star has reached the same maximum luminosity
observed in late 1958 (Evans 1959) that could be close
to the unspotted V magnitude value. The observed colors
can be well matched by assuming a K7 V + M1 V spectral
classification.
Table 4: V magnitude and colors for the c and
ck stars derived via standard stars. Errors are of
the order of 0.01 magnitudes. The symbol ``:" denotes
errors of the order of 0.02 magnitudes
HD 32918 = YY Men is an active giant with the typical
characteristics of FK Com stars (see Cutispoto et al. 1992 and references
therein).
It has shown two of the most intense and longest duration radio flares
(Slee et al. 1987; Bunton et al. 1989)
and probably the longest optical
flare ever recorded for any class of active stars (Cutispoto et al. 1992).
The data obtained over the interval 1-10 Oct. by using the 1.0 m telescope
are shown in Fig. 6 (click here).
Phases are computed using the 9.5476 day
photometric period
inferred by Collier (1982). The V-band light curve is single-peaked and
its maximum is probably close to the stellar unspotted level (cf Fig. 7 (click here)).
Remarkable color variations well correlated with the variability in the
V-band are present and show the star to be redder at light minimum.
HD 155555 = V824 Ara is a rapidly rotating SB2
spectroscopic binary that has been classified as a pre-main sequence object
by Pasquini et al. (1991) and by Martin & Brandner (1995).
It was detected
by the ROSAT (Pounds et al. 1993; Pye et al. 1995)
and EUVE (Malina et
al. 1994) satellites during their all-sky EUV surveys and has been observed
photometrically since late 1979 by several authors (see Cutispoto 1996 and
references therein). The observations presented in Fig. 8 (click here)
were obtained with
the 0.5 m telescope over the interval 7-17 Sep. Any light contribution from
LDS587 B, that lies 33 arcsec apart, was avoided. Phases have been reckoned
from the 1.681652-day orbital period computed by Pasquini et al. (1991). The
light curve is single-peaked and has a different shape and a larger amplitude
with respect to the light curve obtained less than seven months before
(Cutispoto 1996). The colors, whose variations are in phase with the
V-band modulation and show the star to be redder at minimum luminosity,
are consistent with the G5 IV + K0 IV spectral classification reported by
Pasquini et al. (1991). Finally, from the collection of the available
photometry of V824 Ara shown in Fig. 9 (click here), we note that the present observations
show the faintest V brightness ever observed, confirming the presence
of strong variations of the global degree of photospheric spottedness with a
tentative period of at least 13 years.
HD 174429 = PZ Tel is a rapidly rotating late-type
chromospherically active star that has been classified as a very young or
as a pre-main sequence star (Innis et al. 1986). It shows a filled in H
line (Innis et al. 1988) and Ca II H&K emission lines
(Bidelman & MacConnell
1973; Houk 1978; Innis et al. 1988).
The radial velocity data
suggest that the star is single (Balona 1987; Innis et al. 1988).
HD 174429 has
been detected during a microwave survey of southern active stars (Slee et al.
1987) and shows a very high Li abundance (Randich et al. 1993). The optical
variability was discovered by Coates et al. (1980), further photometric data
can be found in Bopp et al. (1986) and Innis et al. (1990).
The observations
presented in Fig. 10 (click here)
were obtained over the intervals 7-17 Sep. and 30 Sep. - 10
Oct. with the 0.5 m and the 1.0 m, respectively; phases have been
computed by using the photometric period of 0.94486-day inferred by
Innis et al. (1990). The light curve has a very low
amplitude and no significant color variations are present. A
collection of the available photometry of HD 174429 is shown in Fig.
11 (click here). From these data it is evident that the mean V
magnitude has been almost constant over the last 12 years. Houk
(1978) classified PZ Tel as a K0 Vp star and our colors are
in agreement with such a classification, with the noticeable
exception of the U-B color that is about 0.15 magnitudes
bluer than expected. However, Randich et al. (1993)
computed for the 
a value of the order of 
, which leads to infer a minimum stellar radius of about
. This radius is too high for a luminosity class V
star, confirming that HD 174429 is probably still approaching
the main sequence.
HD 197481 = AU Mic is a flare star that is also a member of the
BY Dra-type variables. It shows strong emission lines indicative of the
presence of an active chromosphere and transition region (Linsky et al. 1982;
Quin et al. 1993; Houdebine & Doyle 1994
and references therein). X-ray and EUV
emission has been revealed by the Einstein (Golub 1983),
ROSAT (Pounds et
al. 1993; Pye et al. 1995) and EUVE (Malina et al. 1994)
satellites. The optical
variability was discovered by Torres et al. (1972).
The observations presented here were obtained using the
0.5 m telescope over the interval 7-17 Sep. They are shown in Fig. 12 (click here),
where
phases are reckoned from the photometric ephemeris 
(Torres et al. 1972). The colors, that show weak variations,
are consistent with the M1 Ve spectral classification reported by Houk (1982).
In particular, the V-R and V-I modulation is clearly in phase with
the V-band variability.
HD 214479 = FK Aqr is the brightest component of the visual
pair ADS 11854 (= GL 867 AB) and is itself a spectroscopic SB2 binary with an
orbital period of 4.08322 days (Herbig & Moorhead 1965). The combined spectral
type is given as dM2e and M0Ve by Joy & Abt (1974) and Houk &
Smith-Moore
(1988), respectively. FK Aqr has been observed by several authors showing the
classical behavior of flare stars (see Cutispoto 1991 and references
therein). More recently, evidence for X-ray flaring activity has been reported
by Pollok et al. (1991) and the star has been
detected at EUV wavelengths by the ROSAT (Pounds et al.
1993; Pye et al. 1995) and EUVE
(Malina et al. 1994) satellites. The observations
carried out over the interval 7-17 Sep. with the 0.5 m and over the
interval 30 Sep. - 9 Oct. with the 1.0 m are shown in Fig. 13. Any
light contribution from the optical companion GL 867 B = FL Aqr,
that lies 24.5 arsec apart, was avoided. Phases are computed by
using the 4.252 
0.044-day period inferred from a Fourier
analysis of the VRI data. We note that this photometric
period is considerably shorter than the 4.39-day value computed by
Byrne et al. (1987), a difference that is indicative
of the presence of differential rotation. Moreover, the light curve
in Fig. 13 (click here)
is double-peaked, while the one obtained less
that 10 months before (Cutispoto 1995) was
single-peaked. As the data in Cutispoto (1995) were
folded by using the 4.39-day period, it is evident that strong
modifications of the starspots' distribution, and in particular of
the spot latitude, have occurred. The present B-V, V-R
and V-I color indices are clearly correlated with the
V-band modulation, while the more scattered U-B color data seem
to be anticorrelated. The colors are consistent
with the combined M2 Ve spectral classification.
HD 217344 = TZ PsA is an SB1 system with an orbital period of
1.643 days (Balona 1987) showing Ca II H&K emission lines
(Houk 1982). It
was detected during a microwave survey of southern active stars (Slee et al.
1987) and during the Extended Medium Sensitivity Survey by the Einstein
satellite (Gioia et al. 1990). Low Li abundance is reported by
Favata et al.
(1995) and optical variability was discovered by Lloyd Evans & Koen (1987).
The observations carried out using the 0.5 m over the interval 7-14 Sep. are
shown in Fig. 14 (click here).
Phases are reckoned from the 1.648-day photometric period
computed by Lloyd Evans & Koen (1987). The faint optical companion
(
) noted by
Houk (1982) was always included into the diaphragm
of our photometer. The low amplitude light curve appears rather
complicated and weak color variations are present. A collection of
the published photometry of HD 217344 is shown in Fig. 15 (click here).
The few data available do not allow to draw any conclusion on the
presence of a long-term variability of the degree of spottedness.
Assuming the G5 V spectral classification reported by Houk
(1982) for the primary component of HD 217344, the observed
colors are very well matched by considering a G5 V + K7-M0 V
system. It should be noted that, being HD 217344 a SB1 system, the
inferred K7-M0 V component has to be considered as the combination
of the optical companion plus the true physical companion.
However, Favata et al. (1995) computed a 
of
, that leads to a minimum radius for the
primary component of 
i.e. to a luminosity class IV
star. A good match of the observed colors is then obtained by
assuming a G5/6 IV primary component.
Acknowledgements
Stellar activity research at Catania Astrophysical Observatory is supported by the Italian Ministry for Scientific Research and Technology (MURST), the Italian National Council for Research (CNR - Gruppo Nazionale di Astronomia) and the Sicilian Regional Government (Regione Sicilia) that are gratefully acknowledged. The authors thank the referee K.G. Strassmeier for his constructive comments and Ms. G. Santagati for revising the manuscript. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. A special thank is also due to the ESO staff for the collaboration and technical support during the observations.