The active W UMa type binary star V781 Tau revisited

TL;DR

This study presents new photometric and spectroscopic data for V781 Tau, revealing its contact binary nature, period variations, and potential third companion, advancing understanding of W UMa type binaries.

Contribution

The paper provides the first simultaneous analysis of light curves and radial velocities for V781 Tau, confirming its classification and uncovering period variations and a possible third body.

Findings

V781 Tau is a W-subtype medium contact binary with q=2.207.

Detected a secular period decrease likely due to magnetic braking.

Identified cyclic period modulation possibly caused by a third companion.

Abstract

In this paper, new determined BVR_cI_c 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 orbital period change of V781 Tau was also investigated. A secular decrease at a rate of $-6.01(+-2.28)*10^{-8} d/yr and a cyclic modulation with a period of 44.8+-5.7 yr and an amplitude of 0.0064+-0.0011 day were discovered. The continuous period decrease may be caused by angular momentum loss due to magnetic stellar wind. 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.