The (O–C) diagram problem of triple pre-cataclysmic system V471 Tau solved

Abstract

V471 Tau was discovered as a spectroscopic binary by Wilson (General Catalogue of Stellar Radial Velocities, p. 44, 1953). It is the prototype of a post-common envelope system and a progenitor of a cataclysmic binary. The system consists of a cool red dwarf, K2 V, very probably a main sequence star and a hot white dwarf. The object was classified as a close binary with an orbital period of around 0.5 days. Moreover, on the light curve are observed light variations with a period of 191 days, which are connected with an ellipsoidal shape of the red dwarf as well as with the migration of spots on the surface of this cool component. The eclipse of the white dwarf in the binary remains 49 minutes and declines to the minimum and the increase from the minimum takes only 55 s. Such photometric behaviour hinders obtaining good eclipses. In this paper we have obtained during four years seven eclipses with high time resolution with all four contacts to reach precise times of minima. Fortunately these times of the minima show a change of trend in the (O–C) diagram, and we were able to decide about the physical processes responsible for the behaviour of the (O–C) diagram. We showed that this behaviour is caused by a third body in the system with an orbital period of 33.2 years, and its physical and geometrical parameters are presented. For an inclination larger than 35° we get the mass of this body below the stable hydrogen-burning limit and thus most probably the candidate would be a brown dwarf.