A photometric study of 21 Comae Berenices.

Summary. New photometric (B and V) observations of the Ap star 21 Com have been obtained at the Kitt Peak and the David Dunlap Observatories. The observations confirm the 30-minute light variation reported in 1957 by Bahner and Mawridis. The range in V varies from ()W004 to OW019 and the range in (B - V) is very small. The cause of the variation is probably pulsation; this conclusion is based on the similarity between the properties of 21 Com and the corresponding properties of known Sct stars, such as Trumpler 51 in the Pleiades. If this conclusion is correct, then the Ap and Am stars differ in that at least one Ap star pulsates whereas no Am star pulsates. The implications of this difference are discussed briefly The present observations also confirm the longer- period light variation (period, about a day; range about ()W02 in B and V) noted previously. New photometric observations of two other Ap stars (41 Tau and HD 32633) show no evidence for short-period light variations. Key words: photometry - peculiar A stars - Scuti stars