9 Conclusions

We have shown that the Be star $\omega$ Ori undergoes several variations. The star pulsates non-radially with a frequency of 1.03 c d^-1 associated with a NRP mode with l = 2 or 3 and |m| = 2. Some of the ejected material may still be linked to the star and pulsate as well. A second frequency is detected at 0.46 c d^-1 and is attributed to material temporarily orbiting around the star ejected prior to or at the beginning of the campaign. Rotational modulation alone may be ruled out to explain all the observed variations. Both NRPs and rotational modulation of orbiting clouds are needed to explain short-term and mid-term variations in $\omega$ Ori.

We also showed that the cloud was diluted in the middle of the campaign when an outburst occurred, possibly enhanced by another ejection of material. A longer timebase is necessary to investigate whether these kinds of events could be periodic and attributed to beating effects of NRPs.

Acknowledgements

We wish to thank the MuSiCoS 98 team, including the technical staffs and time allocation committees for the telescopes. We are grateful to the French association of amateurs AUDE for providing H$\alpha$ spectra, and to S. Brillant and N. V. Leister for the 2000/2001 NTT and FEROS data. We also thank the referee, D. Baade, for his constructive comments. This research has made use of the Simbad database maintained at CDS, Strasbourg, France.