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Astron. Astrophys. 325, 167-177 (1997)

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Geneva photometry in the young open cluster NGC 6231 *

D. Raboud 1, N. Cramer 2 and P.A. Bernasconi 2

1 Institut d'Astronomie de l'Université de Lausanne, CH-1290 Chavannes-des-Bois, Switzerland
2 Observatoire de Genève, CH-1290 Sauverny, Switzerland

Received 10 January 1997 / Accepted 11 March 1997


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 [FORMULA] ['arc], extending the Seggewiss area ([FORMULA] ['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 [FORMULA] 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.

Key words: open clusters: individual: NGC 6231 – technique: photometry – stars: formation – stars: pre-main sequence

* Tables 3, 4 and 5 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( or via http://cdsweb.u-strasbg.fr/Abstract.html

Send offprint requests to: Didier.Raboud@obs.unige.ch

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998