Long-Term Luminosity Variations and Period Changes In V-471 Tauri

The white-dwarf red dwarf eclipsing binary V 471 Tauri has been observed in blue and yellow lights for twenty years. The behaviour of the mean brightness of the system following its discovery has been studied thoroughly. The system has been brightened more. or less regularly about 0.2 mag in both colours over an interval of 23 yr. This nearly linear increment in the mean brightness has been extracted from the observed mean brightness and a cyclic change has been found. The average period of this cyclic change is 6 yr with an uncertainty of 1 yr. A total of 154 timings of mid-eclipse has been obtained up to date. To explain the almost sinusoidal O-C curve with an amplitude of 200 s the light-time effect has been assumed. The model features a constant period for the eclipsing pair with the third-body orbit of period 24.6 yr, eccentricity 0.614 and a sin i = 3.48 10(7) km. For the inclination above 30-degrees the computed mass leads to a substellar mass. After subtracting the time delay due to orbiting around a third body from the O-C curve a cyclic variation has been obtained. The average cycle is 5 yr with an uncertainty of 0.5 yr. The average amplitude of these changes is now about 45 s. The changes in the orbital period seem to be related to the system's mean brightness variations. The closeness of the periods for the luminosity variations and the orbital period modulation may be taken as the evidence which supports the theory developed by Applegate recently. The maximum of the O-C curve coincides with the maximum luminosity which suggests that the outside spin of the K2 dwarf should be slower than that of inside.