¹ Astronomical Institute, Academy of Sciences, CZ-25165 Ondrejov, Czech Republic
² European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching b. Muenchen, Germany
³ Landessternwarte Königstuhl, D-69119 Heidelberg, Germany
⁴ European Southern Observatory, Casilla 19001, Santiago 19, Chile

The rapid line-profile variability of the early-type and pole-on Be star  CMa between 1996 and 2002 is characterized across the complete optical spectrum, for quiescent phases as well as for outbursts. Owing to different and changing line-profile variability patterns, amplitudes and -velocities are different from line to line and are variable on a time scale of months. A comprehensive time series analysis was performed on the modes of a set of selected lines (after individual seasonal normalization to avoid biases). At a high level of confidence, only the well-known 1.37-d period could be found in photospheric lines not contaminated by the disk. Outside major outbursts, when the star is at its photometric ground state, the phase coherence of the variability is very robust. During strong outbursts, when the star is visually bright, the period may either be very slightly different or phase jumps may occur. The present observations do not have the sampling necessary to distinguish between these possibilities. Harmanec's (1998) report of continuous, cyclic period variations cannot be confirmed. Arguments are presented that temporary period changes may be related to interactions between the photospheric non-radial pulsation and the disk when (during outbursts) these two domains are in contact with one another. This result does not seem to be an artifact of the also previously reported transient periodicities near 1.49 d, which are prominent during outbursts and seem to be anchored in the exo-photosphere. However, if not properly taken into account, they may easily lead to false conclusions about multiple or variable periods. In the Be star µ Cen, which has a similar spectral type, outbursts are triggered by the beating of two or more non-radial pulsation modes (Rivinius et al. 1998b). Since  CMa, too, undergoes outbursts although its photospheric variability is single-periodic, the case of µ Cen cannot be generalized to the activity of all early-type Be stars or to the Be phenomenon at large.

Accepted by A&A