9 Discussion

The results of our H$\alpha$ observations indicate that previous searches detected virtually all bright Be stars in NGC663. As could be expected, our search resulted in new detections of H$\alpha$emission in late B-type stars only. The search was complete down to the A5-type stars in the cluster and revealed that Be stars are most populous in the range corresponding roughly to the spectral types between B0 and B3 (see Fig. 4). Although Be stars were also found among cluster members with the spectral type later than B3, their fraction amounts to only 7 $\pm$ 3%. Such a distribution of Be stars with spectral type was already known (Schild & Romanishin 1976), but was derived from a small sample of stars and searches which were not complete. The case of NGC663 confirms the dependence of Be star distribution as a function of spectral type, but more clusters of different ages need to be observed in order to come to a reliable conclusion as to this effect. H$\alpha$ observations of other northern clusters rich in Be stars are now underway.

In terms of the $(R-I)_{\rm C}$ colour-index, Be stars in NGC663 are, on average, 0.04 mag redder than non-emission stars which define the cluster main sequence. This additional reddening, presumably of circumstellar origin, is observed in almost all open clusters containing Be stars.

Acknowledgements

We are indebted to Prof. M.Jerzykiewicz for critical reading the manuscript. We also thank the referee, Dr. J.Zorec, for his valuable comments. This research was supported by the KBN grant No. 2P03D2909.