ON THE EPHEMERIS OF THE PULSATING HYDROGEN-DEFICIENT STAR V652 HER

Some new timings of maxima of the pulsating hydrogen-deficient star V6 52 Her (BD + 13 degrees 3224) obtained during 1991-95 are reported. Th e data show that the ephemeris for times of maxima can no longer be re presented by a cubic polynomial, and that a quartic term is necessary. The star could have brightened by up to 0.008 mag over the last decad e, although this is not firmly established (the apparent change could also be due to small zero-point effects, for example). The shape of th e light curve appears essentially unaltered over the same time interva l; any changes are less than similar to 0.003 mag in size. Attempts ha ve been made to understand the changing ephemeris in terms of changing stellar radius (R), mass (M) and pulsation constant (Q). A satisfacto ry match to the observations is obtained by a function including the f irst three derivatives of radius, R over dot, R double over dot and <( R)triple over dot. The addition of a small mass-loss term does not sig nificantly improve the fit. With the available data we cannot rule out (or distinguish between) small values of M over dot or Q over dot, bu t the values derived for the radius derivatives should stringently con strain pulsation models for V652 Her, and suggest extremely rapid evol ution not yet accounted for in non-linear radial pulsation calculation s.