5 Summary and concluding remarks

Our time series of spectra of the O6.5ef star BD  $+60^{\circ }$ 2522 show that the double-peaked He II$\lambda$ 4686 emission line undergoes strong profile variability on time scales of 2-3 days similar to the variations seen in two other Oef stars, $\zeta$ Pup and $\lambda$ Cep. Both the time scale as well as the pattern of the variations in BD  $+60^{\circ }$ 2522 turn out to be epoch dependent, suggesting that the phenomenon is not related to the sole effect of stellar rotation. On the other hand, the absorption lines display line profile variability on time scales of a few hours which are probably indicative of non-radial pulsations. Dedicated observations with a better time resolution are needed to confirm the latter finding and to quantify the time scales of these pulsations.

It seems very likely that the wind of BD  $+60^{\circ }$ 2522 is not spherically symmetric. Out to the formation region of the He II$\lambda$ 4686 line, the wind appears rather as some sort of corotating equatorially confined envelope. In this context, it would be extremely interesting to investigate the possible presence of a stellar magnetic field in BD  $+60^{\circ }$ 2522. Evidence for a stellar magnetic field might be obtained through the detection of a synchrotron (i.e. non-thermal) radio emission. A detection of a non-thermal radio emission from any of the Oef stars might therefore be of primary importance for our understanding of the Oef phenomenon. We note however, that at least for $\zeta$ Pup, the VLA observations by Bieging et al. (1989) revealed a purely thermal radio spectrum.

Acknowledgements

We wish to express our thanks to Dr. Jean Manfroid for taking some of the spectra of the September 2000 campaign and to the referee Dr. P.S. Conti for the careful reading of the manuscript. We are greatly indebted to the Fonds National de la Recherche Scientifique (Belgium) for multiple assistance including the financial support for the rent of the OHP telescope in 1999, 2001 and 2002 through contract 1.5.051.00 "Crédit aux Chercheurs'' FNRS. The travels to OHP for the observing runs were supported by the Ministère de l'Enseignement Supérieur et de la Recherche de la Communauté Française. This research is also supported in part by contracts P4/05 and P5/36 "Pôle d'Attraction Interuniversitaire'' (SSTC-Belgium) and through the PRODEX XMM-OM Project. We would like to thank the staff of the Observatoire de Haute Provence for their technical support during the various observing runs. The SIMBAD database has been consulted for the bibliography.