MAPPING THE BETA-CEPHEI INSTABILITY STRIP - PHOTOMETRIC VARIABILITY OF STARS IN THE CENTRAL PART OF THE CYGNUS OB2 ASSOCIATION

The central part of the Cygnus OB2 association was searched for the pr esence of short-period hot pulsators. Out of 16 O-type stars in the fi eld, five turned out to be variable. None of the O-type variables show ed short-period variations characteristic for the beta Cephei-type sta rs. However, two beta Cephei stars were found among the early B-type m embers of the association. Their pulsation periods are too long to be in agreement with the average association age. We note that the discre pancy can be explained by non-coeval star formation, which was already suggested by other authors from the investigation of the colour-magni tude diagrams of the association. We found 29 new variables in all fie lds searched by us. About half of these are Cygnus OB2 members. By mea ns of H alpha photometry, we confirmed the presence of H alpha emissio n in two stars and found one new Be star. A weak Ha emission is probab ly present in all observed O-type supergiants and giants. We also give the new BVRIH alpha photometry for almost 300 stars. This multicolour photometry is used to determine a dereddened colour-magnitude diagram for the association and make a reddening map of the observed field. T he extinction in the field, expressed as the total absorption in the J ohnson V filter, A(v), ranges from 4.1 to 7.0 mag; the average Av amou nts to 5.8 mag. One O-type star is probably a background object.