Simultaneous photometry and spectroscopy of the Be star 28 (omega) CMa - I. Observational evidence of the periodic components of rapid variability

We analyse the periodic variability of the Be star 28 (omega) CMa. The data consist of 275 new He I 6678 line profiles of high spectral and temporal r esolution, and new Stromgren and Geneva photometry. The photometric analysi s has been extended by including previously published data to cover an inte rval of 16 yr. The principal spectroscopic period of 1.37 d is confirmed by analysis of He I 6678 modes and moments. There is evidence to show that a single period i s insufficient to explain the variation in the line wings. In particular, t he variation of the second moment, which samples the line wings, is best de scribed when a second period is included. The photometric data have been divided into 10 sets in which the time is su fficiently contiguous for period analysis. These sets have been smoothed to remove as well as possible the long-term irregular variations which are al ways present in Be stars. The residuals have then been analysed for coheren t periodicities using several techniques. Evidence for one or the other of the two known spectroscopic periods can be found in individual data sets. B y an analysis in which two periods are assumed, it is possible to recover b oth the principal spectroscopic period and the satellite period of 1.46 d. Unlike in the He I 6678 line profiles, the latter period seems to be most o ften present in the light variations. The amplitude of the periodic compone nts of the Light variation is only a few millimagnitudes, in contrast to th e very large radial velocity amplitude. We find that the photometric amplit udes of the two periodic components, and their ratio, vary smoothly on a ti me-scale of years. The amplitudes are larger in seasons when the mean brigh tness is higher.