CAOSP abstracts, Volume: 41, No.: 1, year: 2011

Abstract: V471 Tau is a semi-detached binary classified as a pre-cataclysmic system of a post-common envelope consisting of a white dwarf and a main sequence star of the spectral type K2 V. The eclipsing system is characterized by very fast decreases to the minima as well as the steep increases from minima. The change of brightness caused by the eclipse of the white dwarf lasts only 55 seconds and the total eclipse from the whole orbital period (0.5 day) takes 49 minutes, which is 0.07 of the phase. This behaviour of the system complicates obtaining of eclipses of the high quality from which we can determine the time of minima with expected precision. In the past we tried to observe similar fast changes of brightness by high speed photometry at the expense of a signal-to-noise ratio. In this work we present minima of V471 Tau obtained by CCD photometry that enabled to obtain seven time minima with quite high precision. Whereas the (O—C) diagram of V471 Tau shows long term and gentle changes of values (on average 19 seconds per year), it was therefore necessary to use as the basic time system the heliocentric ephemeris Julian date. We used the time of minima corrected by the difference between the two time systems to construct the (O—C) diagram. Whereas during our observations the (O—C) diagram trend changed we could determine the right model of V471 Tau system, interpret the changes of the (O—C) diagram by a third body in the system and determine the geometry of its orbit as well as the mass function.