The authors thank Wilhelm Seggewiss for allocating observing time at the telescopes of Hoher List Observatory, Santi Cassisi for providing the isochrones necessary for our study and Andrea Dieball and Klaas S. de Boer for carefully reading the manuscript. J.S. thanks Georg Drenkhahn for his valuable hints concerning the maximum likelihood analysis software. M.A. and J.B. acknowledge financial support from the Deutsche Forschungsgemeinschaft under grants Bo 779/21 and ME 1350/3-2, respectively. This research has made use of NASA's Astrophysics Data System Bibliographic Services, the CDS data archive in Strasbourg, France, and J.-C. Mermilliod's WEBDA database of open star clusters.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000