MOST photometry and DDO spectroscopy of the eclipsing (white dwarf + red dwarf) binary V471 Tau

Abstract

The Hyades K2 V + WD system 471 Tau is a prototype post-common envelope system and a likely cataclysmic binary progenitor. We present 10 days of nearly continuous optical photometry by the MOST (Microvariability and Oscillations of Stars) satellite and partly simultaneous optical spectroscopy from DDO (David Dunlap Observatory) of the binary. The photometric data indicate that the spot coverage of the K dwarf component was less than observed in the past, suggesting that we monitored the star close to a minimum in its activity cycle. Despite the low spot activity, we still detected seven flarelike events whose estimated energies are among the highest ever observed in V471 Tau and whose times of occurrence do not correlate with the binary orbital phase. A detailed O - C analysis of the times of eclipse over the last ~35 years reveals timing variations which could be explained in several ways, including perturbations by an as-yet-undetected third body in the system or by a small orbital eccentricity inducing slow apsidal motion. The DDO spectra result in improved determinations of the K dwarf projected rotation velocity, V[subscript K] sin i = 92 km s[superscript -1], and the orbital amplitude, K[subscript K] = 150.5 km s[superscript -1]. The spectra also allow us to measure changes in H[alpha] emission strength and radial velocity variations. We measure a larger H[alpha] velocity amplitude than found previously, suggesting that the source of the emission in V471 Tau was less concentrated around the sub-white dwarf point on the K star than had been observed in previous studies.