We thank Lucien Kuiper, Frits Paerels and Karel van de Hucht for useful information and discussions. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center and the ROSAT Archive at the Max Planck Institut für Extraterrestische Physik. This work was financially supported by NWO, the Netherlands Organization for Scientific Research.
© European Southern Observatory (ESO) 1998
Online publication: September 30, 1998