P.K. thanks H.Henrichs and P.Ghosh for helpful discussions. I thank E.P.J. van den Heuvel for reading the article. This research was supported in part by the Netherlands Organisation for Scientific Research (NWO) through Spinoza Grant 08-0 to E.P.J. van den Heuvel. The ROSAT project is supported by the Max-Planck-Gesellschaft and the Bundesministerium für Forschung und Technologie (BMFT).
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999