4 Results and discussion

The new multi-colour photometric observations reported here not only reflect the present status of activity of these stars, but it also would prove to be very useful for future long-term photometric study. Our observations cover the entire phase of light variations of these stars and this would be quite useful for starspot modeling and other spot activity diagnostics.

  

Figure 3: BVR light curves of RS CVn star IM Peg during epoch 1995-96

4.1 V711 Tau

The variability of non-eclipsing double-lined spectroscopic binary V711 Tau is being continuously monitored photometrically as well as spectroscopically since its discovery as a variable star by Landis & Hall (1976). Recent papers containing extensive photometric analysis of this star are by Mohin & Raveendran (1993a) and Henry et al. (1995). The knowledge of the orbital inclination is very important for the latitude determination of starspots, and this parameter(i = 30$^\circ$) has been obtained with high accuracy by Fekel (1983) and Donati et al. (1990). In the past, several attempts have been made to fit photometric light curve of this star in terms of large dark spots (Dorren et al. 1981; Kang & Wilson 1989; Henry et al. 1995 and references therein). Here, BVR photometry of V711 Tau obtained during 1996-97 for total 42 nights is presented. A remarkable variation in the light curve within a year can be seen from Fig. 1. Differential amplitude jumps from 0.07 to 0.18 mag, whereas minima still persist at 0.4 phase. Modeling the light curves with two spot model indicate that nearly two equal sized spots at relatively low latitude could explain the observed light variation during the epoch 1996.09, whereas unequal sized spots lying in opposite hemispheres are needed to reproduce the light curve for the epoch 1997.08. The brightness at minimum and maximum seem to follow the suggestion of Henry et al. (1995) i.e. it decreases at minimum and increases at maximum. This behaviour in turn reflects that changes in the amplitude are mainly due to redistribution of spots on the stellar surface rather than overall changes in the level of spottedness. The coverage of the fractional spot area shows close agreement with previously reported spot area obtained by modeling of photometric light curves with two spot model (Rodono et al. 1986). We have not found explicit variation in (B-V) and (V-R) colour indices for this star.

  

Figure 4: BVR light curves of RS CVn star II Peg during epoch 1995-96

4.2 UX Ari

The well known non-eclipsing active binary UX Ari has been frequently observed at different wavelengths by several investigators in the past two decades (see e.g. Raveendran & Mohin 1995; Padmakar & Pandey 1996). The double-lined spectroscopic binary system UX Ari comprises a hot G5 V and an active cool K0 IV stars. From continuous monitoring of UX Ari during 1988-89, Rodono & Cutispoto (1992) recorded a clear indication of short term variation in its light curve. We used i = 60$^\circ$ for this star while modeling its light curves (Poe & Eaton 1985). BVR observations on UX Ari for the period from Nov. 1995 to Feb. 1997 are reported here. A visual

inspection of its light curve (Fig. 2) reveals the existence of two maxima separated from each other by 0.37 in phase, and further that the longitudinal separation is maintained in the next epoch too. Spot parameters as derived from starspot models seem to indicate movement of two spots close to each other and also towards lower longitudes. This apparently contradictory results may be due to unreliable determination of the spot latitudes through starspot modeling. The value of maximum brightness during the epoch 1997.08 is found to be $\sim$0.82 mag. This value of maximum brightness is the same as obtained at the mean epoch 1989.10 by Raveendran & Mohin (1995). We have also recorded variation in (B-V) colour index with an amplitude of $\sim$0.05 mag. Our observations confirm the definite anti-correlation between $\Delta V$ light curve and (B-V) colour index curve, as reported by several investigators (Zeilik et al. 1982; Rodono & Cutispoto 1992; Raveendran & Mohin 1995). The unexpected anti-correlation in (B-V) and constancy in (V-R) colour indices may be useful to solve the ambiguity in determining whether the variation in (B-V) is due to flaring and facular origin (Rodono & Cutispoto 1992) or whether it is merely due to fractional contribution of the hot star in the blue region to the composite light (Raveendran & Mohin 1995).

  

Figure 5: BVR light curves of RS CVn star $\sigma$ Gem during epoch 1996-97

4.3 IM Peg

IM Peg is one of very active bright members of long period RS CVn group. The coordinated photometry and spectroscopy of this star clearly indicate anti-phase variation of H$_ {\alpha}$ emission line (Dempsey et al. 1996). The light curves obtained by Eaton et al. (1983) and Strassmeier et al. (1989) showed remarkable variation in the amplitude (from 0.11 to 0.24 mag) without showing any significant variation in the shape. Our observations for the period 95-97 support this picture. For the epoch 1995.89 our observations coincide with those of Strassmeier et al. (1997). Both observations give similar results except the difference in the phase of minimum light which is the result of using different ephemeris. The orbital inclination i = 60$^\circ$ was adopted for IM Peg from Poe & Eaton (1985). The amplitude in V band for the epoch 1996.9 is $\sim$ 0.38 mag, which is significantly larger than that found by previous observers. Our photometric observations indicate movement of light minima towards a lower phase. Although, the observed light curves appear nearly symmetric, two spots were needed to fit them, perhaps an account of the deep minima. Our two years observations reveal a remarkable variation in (B-V) as well as (V-R) colour indices for the epoch 1995.9, however, similar kind of variation in colour indices are not seen during the next observing epoch (see Fig. 3). We have modeled the (V-R) colour index curve to obtain $\Delta T$, the temperature difference between the photosphere and the spot, which comes out to be $1130\pm 35$ K , whereas Poe & Eaton (1985) obtained $\Delta T$ $\sim$ $920\pm100$ K for this star.

  

Figure 6: BVR light curves of RS CVn star $\lambda$ And during epoch 1995-96

4.4 II Peg

It is one of the most active RS CVn stars, and it has been frequently observed by several investigators at different wavelengths (Rodono et al. 1986; Strassmeier et al. 1988; Mohin & Raveendran 1993b; Henry et al. 1995 and references therein). Simultaneous photometric and spectroscopic observations indicate strong correlation between light variation and the strength of TiO bands , whereas H$_ {\alpha}$ emission line strength was found to be anti-correlated with the photometric variation (Mohin & Raveendran 1993b). Following Poe & Eaton (1985) and Henry et al. (1995) we have adopted a value of i = 34$^\circ$ for the orbital inclination. Our results on BVR observations for II Peg carried out during Nov. 1995 to Feb. 1997 are displayed in Fig. 4. Seasonal variations in the amplitude, shape and phase of minimum light are evident from this figure. The light curve which showed double peak during the epoch 1995.89, turned in to a near sinusoidal with high amplitude for the epoch 1996.89. The (B-V) and (V-R) colour indices seem to be correlated with visual amplitude V i.e. the star was bluer at maxima and redder at minima. However, the brightness at the maximum and the minimum for II Peg does not follow Henry's et al. (1995) picture. The modeling of the (V-R) colour index curve for the epoch 1985.89 gives the value of $\Delta T \sim 740\pm45$ K. The derived value of our $\Delta T$ falls quite close to the lower limit of the range of values ($\sim$700 to $\sim$1000 K) obtained by other investigators (Poe & Eaton 1985; Vogt 1981; Byrne et al. 1987) from modeling of the (V-R) colour index curves. Recently O'Neal, Saar and Neff (1998) have reported the first spectroscopic evidence of the multiple spot temperature for this star. From TiO molecular band observations they have found that spot temperature varied between $\sim$3350 to 3550 K during the epoch of September 1996 to October 1996, whereas the starspot filling factor was constant ($f_{\rm s}\sim55\%\pm5\%$). Our photometric observations carried out during nearly the same epoch (1996.89) give different results probably due to the different modeling techniques applied.

4.5 $\sigma$ Gem

It is one of the brightest RS CVn binary star. A systematic photometric study of this single-lined spectroscopic binary has been carried out by Strassmeier et al. (1988), and by Henry et al. (1995). Dempsey et al. (1992) introduced a cross correlation technique to combine informations contained in the line profiles with those of the broad band optical photometry for a reliable determination of spot latitude and other spot parameters. The cross correlation technique indicates that spots occupy relatively higher latitude regions ($\sim$ 30$^\circ$ to 60$^\circ$). In the past all light curve have been modeled using i = 60$^\circ$ (Strassmeier et al. 1988; Henry et al. 1995), therefore, we also used same value while modeling its light curves. Our result of BVR observations is shown in Fig. 5. A substantial variation in the light curves of $\sigma$ Gem within a year is discernible from the figure. The light curve with single minimum for the epoch 1996.06 turns into a double peaked symmetric one during the next observing epoch, 1997.14. There is too small variation in the colour indices of this star to allow us to determine reliable values of the spot temperature. The brightness at minimum and maximum are exactly the same as found earlier by Henry et al. (1995).

4.6 $\lambda$ And

It is among the brightest (V=3.82) of all known chromospherically active binaries discovered so far. Generally most of RS CVn binaries are found to be rotating synchronously (Hall & Henry 1990), but $\lambda$ And reveals very large asynchronous ($P_{\rm rot}/P_{\rm orb}\sim 2.6$) motion which is rather unusual among RS CVn binaries. $\lambda$ And is a source of strong Ca II H and K and H$_ {\alpha}$ emission lines and these are found to be strongly correlated with the optical light variation (Henry et al. 1995). Historically, the light variability of this star was recorded in the mid 1930s, and subsequently observed by different groups (Bopp & Noah 1980; Dorren & Guinan 1984; Hall et al. 1991; Henry et al. 1995 and references therein). The orbital inclination of the $\lambda$ And is not a well-known quantity, we used i = 60$^\circ$ for spot modeling following Henry et al. (1995). Our BVR observation were carried out during November 1995 to January 1996 is shown in Fig. 6. Previous observations show a variation in V amplitude between 0.05 to 0.23 mag. This in turn implies that during our observations $\lambda$ And was in the stage of moderate activity . The extracted spot parameters from the observed light curves as given in the Table 2, favour the presence of one large spot and another small starspot laying at relatively high latitude to explain the observed light modulation. Our results do not show any significant variation in (B-V) colour index, whereas a marginal variation in (V-R) colour index can be seen from Fig. 6. Modeling of (V-R) colour index curve gives a value of $\Delta T\sim800\pm30$ K while Poe & Eaton (1985) obtained $\sim$$1050\pm100$ K for the of epoch 1980.7.

Acknowledgements

We are thankful to the Head, School of Studies in Physics for providing the observing facilities, and to IUCAA for providing library and computing facilities where a part of the work was done. We also express our sincere thanks to Dr. K. Oláh who refereed the paper and offered her critical comments which led to significant improvements in the present paper. Thanks are also due to Mr. Dhanedra for his assistance during the course of observation. We would like to express our sincere thanks to CSIR New Delhi, India for financial support through a project grant No. 03(0769)/94/EMR-II.