We thank G.J. Ferland for providing the code Cloudy and the referee, A.F.J. Moffat, for a careful reading of the manuscript. J.G. and St.K. are supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF/DLR) under contract Nos. FKZ 50 QQ 9602 3 and 50 OR 9306 5. GT acknowledges CONACYT grant 25454-A. The ROSAT project is supported by BMBF/DLR and the Max-Planck-Society. This research has made use of the Simbad database, operated at CDS, Strasbourg. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166.
© European Southern Observatory (ESO) 1999
Online publication: June 30, 1999