2002A&A...392..535R

We report additional observations of the phase dependent behavior of the chromospheric Hα feature associated with the K-dwarf component of V471 Tau. Previously, all such observations have shown a sharply defined maximum of emission near the middle of the transit phase of the white dwarf across the disk of the K-dwarf. The precision and the reproducibility of that phase lock led many previous observers to conclude that the observed emission was due to chromospheric re-processing of ultraviolet radiation incident upon the K-dwarf from the white dwarf. Comparing these observations with previously published observations, we show that the emission strength has been steadily diminishing between epoch 1985 and epoch 1992 whence it has essentially vanished. The ultraviolet flux from the white dwarf is constant so the resulting re-processed chromospheric emission cannot be time dependent. This leads to the conclusion that most of the emission has been due to magnetically active regions located preferentially at the sub-stellar point of the white dwarf. We propose that tidal coupling in this close binary system preferentially affects the transfer of magnetic energy, coupling the internal dynamo of the K-dwarf to the outer envelope, giving a preferred longitude to magnetic activity. We further suggest that this mechanism may be common in most short period binary systems and in many cases may be the dominant mechanism causing the observed Hα behavior.