1 Introduction

The differences in Be-star fractions in open clusters are still not well understood. Some investigators postulate age (Fabregat & Torrejón 2000) and metallicity (Maeder et al. 1999) effects. In order to study this problem, the Be star fractions have to be known in many clusters and in the whole range of B spectral types. Unfortunately, the observed fractions of Be stars are very often systematically biased because the searches are not complete. This is mainly true in the case of spectroscopic searches which, for practical reasons, were usually confined to the brightest and most well-isolated stars in a cluster. As a consequence, Be star fraction for late B-type stars was often underestimated. The situation was improved when the era of CCD H$\alpha$ photometry began. Although with this technique faint emission cannot be detected, it works well in crowded fields and needs much less telescope time than does spectroscopy. Even with a small-size telescope, late B-type stars in many open clusters can be easily reached, so that complete information on the Be star content can be obtained.

Detection of Be stars by means of the CCD photometry is usually carried out with a filter centred on H$\alpha$ and two other filters located in the Paschen continuum (see, e.g., Grebel 1997). It is even better when two filters of different width centred on H$\alpha$ are used, because they may define a photometric index independent of the interstellar extinction (see, e.g., Goderya & Schmidt 1994). Such a pair of H$\alpha$ filters was used in our observations.

NGC663 is a young galactic open cluster very rich in Be stars. According to Sanduleak (1979), it contains over twenty Be stars, that is, more than one third of all its early B-type stars. This is why this cluster was chosen as one of the first targets in our Be star survey among northern open clusters. Within this search, we have recently discovered that NGC7419, another open cluster of similar age, contains over thirty Be stars (Pigulski & Kopacki 2000). Our project is also complementary to the ongoing search for variables of type B in young open clusters (see Pigulski et al. 2000 and references therein).