1998A&A...332..155S

We present the photometry of a month-long international campaign on the variable Be star ο Andromedae. Excellent time coverage and photometric precision permit a critical comparison for the first time between the pulsational and the rotational modulation hypotheses. A multiperiodic analysis of data taken many years apart shows sets of close frequencies. The amplitude ratio between the ultraviolet and visible variations is what is expected for early-type star pulsation. But, the total amplitude and the order of importance of the frequencies is very different between observation campaigns. A simple double wave periodic curve accounts for most of the light variation: a rotation/modulation model is considered, with activity variations in or just above the photosphere. Any model must explain the observed changes in the amplitudes of the frequence corresponding to the period and its first harmonic. A very simple model with two stable photospheric activity "features" is insufficient to explain the small variations observed around the mean values of the period and its light amplitude. Thus we propose that the photosphere, which is very probably oblate and seen almost equator-on, is divided into zonal bands undergoing differential rotation.