The active W UMa type binary star V781 Tau revisited

In this paper, new determined BVRcIc light curves and radial velocities of V781 Tau are presented. By analyzing the light curves and radial velocities simultaneously, we found that V781 Tau is a W-subtype medium contact binary star with a mass ratio of q=2.207±0.005 and a contact degree of f=21.6(±1.0)%. The difference between the two light maxima was explained by a dark spot on the less massive primary component. The change of the orbital period of V781 Tau was also investigated. A secular decrease at a rate of ‑6.01(±2.28)×10‑8d/yr and a cyclic modulation with a period of 44.8±5.7yr and an amplitude of 0.0064±0.0011day were discovered. The continuous period decrease may be caused by angular momentum loss due to a magnetic stellar wind. The Applegate mechanism failed to explain the cyclic modulation. It is highly possible that the cyclic oscillation is the result of the light travel time effect by a third companion.