The ROSAT project is supported by the Max-Planck-Gesellschaft and the Bundesministerium für Forschung und Technologie (BMFT). This research made use of the COSMOS digitized optical survey of the southern sky, operated by the Royal Observatory Edinburgh and the Naval Research Laboratory, with support from NASA. This research has made use of the SIMBAD data base operated at CDS, Strasbourg, France. We thank K.S. de Boer and U. Klein for critically reading the manuscript. We thank an anonymous referee for useful comments.
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000