Most of the low-resolution spectroscopy needed to restrict the candidate sample was performed by W. Hoff. We are indebted to his work. Thanks go also to W. Voges, W. Pfau, R. Neuhäuser, T. Henning and M. Fernandez. The ROSAT project is supported by the German Government (BMBF/DLR) and the Max-Planck Society.
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999