2010A&A...521A..36M

We attempt to establish the real nature of the orbital period variation and its relation to the spot activity of V711 Tau, and determine why the (B-V) colour of the star appears to be nearly independent of its V magnitude. We wish to verify whether existing predictions in the literature for the long-term spot activity of the star are true or not by making extended photometric observations, and whether the broad component of Hα emission originates in more localised active regions, as suggested by some chromospheric models, by searching for any correlation between the base-line width of the emission and the light modulation. We obtained new radial velocities of the G-type component of V711 Tau on 42 nights during 2004-09, and BV photometry of V711 Tau on 202 nights during the years 1993-2009. We measured the equivalent widths of two well resolved lines of the G-type component from 13 of the spectra obtained by us. We also measured the equivalent widths and base-line widths of Hα emission from 21 spectra obtained during 2008-09. We analyse these along with relevant information available in the literature. The available radial velocity data of V711 Tau are consistent with a sinusoidal modulation of its orbital period; the period of modulation is found to be 36.3±1.9yr. The fractional light-loss over a photometric cycle is found to vary with an average period of 14.1±0.3yr. It appears that physical processes linked to the magnetic activity of the spotted star are responsible for the variation in the orbital period. The excess reduction in the B band flux relative to that in V band caused by spot activity is almost compensated for by the fractional increase in the contribution by the hotter companion to the total light in the blue spectral region, and thereby makes the (B-V) colour of the binary system nearly independent of its V magnitude. We find that the ratio of the radii of the components derived from their v sin i values, which is usually quoted in the literature, is substantially larger than that implied by the ratio of their brightnesses in V band. There is a slight indication from the V band data that the spot activity in V711 Tau has a component that is fixed in the orbital frame of reference. The equivalent width and base-line width of Hα emission do not show any obvious correlation with the V band light curve.