The ROSAT mission was supported by the Ministerium für Forschung und Technologie (BMFT, Deutschland) and by the Max-Planck-Gesellschaft. We would like to thank the ROSAT team for processing the observational data. We thank the referee, Dr. N. Jackson, for his advice. We are grateful to be able to make use of the photographic data obtained using The UK Schmidt Telescope, which was operated by the Royal Observatory, Edinburgh, with funding from the UK Science and Engineering Research Council and from Anglo-Australian Observatory. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166.
© European Southern Observatory (ESO) 1997