AC ANDROMEDAE - THE MISSING LINK BETWEEN DELTA-SCUTI STARS AND CLASSICAL CEPHEIDS

The triple-mode pulsating star AC And has periods of approximately 0.7 1, 0.53 and 0.42 d, which have long suggested that it is an unusual (a lso strong-lined) RR Lyrae star. Earlier theoretical work, however, ha s implied that it must have a mass near 3 M., and this has now been gi ven strong support in an exhaustive study by Kovacs & Buchler. The mos t likely interpretation is that AC And is a normal post-main-sequence star in its first and only crossing of the instability strip. Assignin g an absolute magnitude appropriate to an evolving 3-M. model at the o bserved spectral type of about F5, and taking the 0.71-d period as the fundamental radial mode, AC And is found to obey the period-luminosit y relation of delta Scuti stars and classical Cepheids, occupying a po sition on the relation intermediate between those two classes of star. Ways in which this might be verified are considered.