We thank Dr. J. Schmitt for helpful suggestions. We also thank the referee, Dr. T. Simon, for his careful reading of the manuscript and the many valuable suggestions, which definitely helped to improve the paper. This research has made use of the Simbad database, operated at CDS, Strasbourg, France. The Rosat project is supported by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF/DARA) and the Max-Planck-Society. This project is supported by the Deutsche Agentur für Raumfahrtangelegenheiten (DARA) GmbH under contract 50 OR 9612 0. TAF acknowledges support from NASA under grant number NAGW-3160.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998