COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3042 Konkoly Observatory Budapest 3 July 1987 HU ISSN 0374-0676 1986 LIGHT CURVE OF V711 Tau* The intrinsic light variability of the non-eclipsing binary V711 Tau has been observed by several investigators since 1976. The strong and variable emissions from chromosphere, transition region and corona and the short periodic light variability make the system the most suitable for studying the peculiar behaviour of RS CVn binaries. Mekkaden et al. (1982) made a detailed study of the photometric behaviour of V711 Tau during the interval 1976 and 1982 and found that the brightness at the light minimum remained nearly constant and the wave amplitude increased as the brightness at the light maximum increased. The observations of Bartolini et al.(1983) showed that drastic changes in light curves occured within short time scales and interpreted the phenomenon as due to the latitude and longitude migration of spot groups. Rodono et al.(1986) explained the light curves of 1981-82 on the basis of a two-spot model; a large circular spot near the polar region and the other near the equator. V711 Tau was observed with the ESO 50 cm telescope at European Southern Observatory, La Silla, through Stromgren uvby filters during 13 nights in October 1986. The observations were made differentially with respect to the comparison star HD 22484 and all measurements of V711 Tau included the fainter visual companion ADS 2444B. The instrumental magnitudes were transformed to the standard system by observing a sufficient * Based on observations collected at European Southern Observatory, La Silla, Chile. number of uvby standards. The photometric phases were calculated using the ephemeris of Bopp and Fekel (1976). JD = 2442766.069 + 2.83782dE Figure 1 is a plot of y magnitudes. The mean error in y is of the order of 0.005 mag. The Stromgren y magnitudes can be converted to Johnson V values using the relation V = y + 0.015(b-y) - 0.003 (Olsen, 1983). [FIGURE 1] The maximum, minimum and mean brightness are 5.72, 5.82 and 5.77 mag respectively (in Johnson V). The light curve is nearly quasi-sinusoidal and the minimum occurs at 0.5P. the corresponding amplitude is approximately 0.10 mag. The photometric observations by Wacker and Guinan (1986) during September 1985 and January 1986 showed a flat topped maximum and the minimum light occured at 0.41P. The unpublished observations of 1984/85 exhibited a quasi-sinusoidal light curve with minimum near 0.40P. Major changes in locations and areas of spot groups took place within an interval of two years. The shape of the light curve changed from quasi-sinusoidal to flat topped maximum and back to a nearly quasi-sinusoidal one, while the maximum light level decreased from 5.63 mag in 1984/85 to 5.72 mag in 1986 and the phase of the minimum advanced by 0.10P. In a very active binary like V711 Tau, exhibiting abrupt changes in light curve and emission line features, areas of activity and therefore spots appear at several locations on the surface and these spots merge or disintegrate within a short time scale. Hence, though the light variations can be reproduced by a two spot model, the model may not represent the real picture of spot groups that are responsible for the sudden changes in activity. M.V. MEKKADEN 1,2 1 Sternwarte Bonn Auf dem Hügel 71 5300 Bonn, F.R.Germany 2 Indian Institute of Astrophysics Bangalore, India References: Bartolini, C., et al. 1983, Astron.Astrophys., 117, 149 [BIBCODE 1983A&A...117..149B ] Bopp, B.W. and Fekel, F.C.: 1976, Astron. J., 81, 771. [BIBCODE 1976AJ.....81..771B ] Mekkaden, M.V., Raveendran, A.V. and Mohin, S.: 1982, J. Astrophys. Astr., 3, 27. [BIBCODE 1982JApA....3...27M ] Olsen, E.H.: 1983, Astron. Astrophys. Supl., 54, 55. [BIBCODE 1983A&AS...54...55O ] Rodono, M., et al. 1986, Astron.Astrophys., 165, 135. [BIBCODE 1986A&A...165..135R ] Wacker, S.W. and Guinan, E.F.: 1986, I.B.V.S. No. 2903.