Geneva photometry in the young open cluster NGC 6231

We present photoelectric (127 stars) and CCD (168 stars) Geneva photometry for the very young open cluster NGC 6231. We have searched for new cluster members out to a distance of similar to 13 ['arc], extending the Seggewiss area (similar to 8 ['arc]), and we found at least 64 new probable members in this extended field. Differential reddening is clearly measured across the cluster area. We determine the cluster distance (1800 pc) and age (3.8+/-0.6 x 10(6) yr). The probable presence of PMS stars and the consequence of this population on the cluster formation history is analysed. We also found that the O8.5III star S161 is a long term variable and we present its light curve extending over more than 20 years. Finally we discuss the existence of Ap stars in the cluster.