The young open cluster NGC 663 and its Be stars

We present new $BV(RI)_{\rm C}$H α photometry of the young open cluster NGC 663. The H α photometry is complete down to magnitude $R_{\rm C} = 15.4$, corresponding to spectral type A5 for the cluster members. This allows detection of mild and strong H α emission in all B-type stars in the cluster. In addition to the 22 Be stars known in the observed field of NGC 663, we discovered four new faint stars of this type. We find that Be stars in NGC 663 cover the whole range of the B spectral type. They are, however, most populous among stars with spectral types falling in the range between B0 and B3, where their fraction amounts to 31 ± 8% . Among B-type stars later than B3, Be stars are much less abundant: only 7 out of 101 observed stars, that is, 7 ± 3% , were detected. About 70% of the observed Be stars in NGC 663 show detectable variations of light. In the time interval covered by our observations, the ranges of the largest variations reach 0.4 mag in the I C band. By means of the isochrone fitting, we derived the cluster distance of 2.1 kpc, age of 20-25 Myr, and the mean colour excess $E(R-I)_{\rm C}$ = 0.54 mag, with a ~0.1 mag scatter due to differential reddening.

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