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(abridged) There is considerable interest in the possible interaction betweenparent stars and giant planetary companions in 51 Peg-type systems. Wedemonstrate from MOST satellite photometry and Ca II K line emission that therehas been a persistent, variable region on the surface of tau Boo A whichtracked its giant planetary companion for some 440 planetary revolutions andlies ~68deg (phi=0.8) in advance of the sub-planetary point. The light curvesare folded on a range of periods centered on the planetary orbital period andphase dependent variability is quantified by Fourier methods and by the meanabsolute deviation (MAD) of the folded data for both the photometry and the CaII K line reversals. The region varies in brightness on the time scale of arotation by ~1 mmag. In 2004 it resembled a dark spot of variable depth, whilein 2005 it varied between bright and dark. Over the 123 planetary orbitsspanned by the photometry the variable region detected in 2004 and in 2005 aresynchronised to the planetary orbital period within 0.0015 d. The Ca II K linein 2001, 2002 and 2003 also shows enhanced K-line variability centered onphi=0.8, extending coverage to some 440 planetary revolutions. The apparentlyconstant rotation period of the variable region and its rapid variation make anexplanation in terms of conventional star spots unlikely. The lack ofcomplementary variability at phi=0.3 and the detection of the variable regionso far in advance of the sub-planetary point excludes tidal excitation, but thecombined photometric and Ca II K line reversal results make a good case for anactive region induced magnetically on the surface of tau Boo A by its planetarycompanion.

MOST detects variability on tau Bootis possibly induced by its planetary companion