We thank the anonymous referee for valuable comments that have lead to the improvement of this paper. We acknowledge the use of the Starlink node at the University of Birmingham and thank the developers and maintainers of the X-ray analysis package ASTERIX. DKS and IRS gratefully acknowledge PPARC funding. This research has made use of data obtained from the UK ROSAT Data Archive Centre at the Department of Physics and Astronomy, Leicester University, UK, and the SIMBAD astronomical database at the CDS, Strasbourg.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998