E. L. gratefully thanks the Swiss National Science Foundation for financial support and, in particular, Professor R. Buser and the Astronomisches Institut der Universität Basel for their hospitality. We acknowledge the referee, Dr Pols, for helpful comments which have improved the clarity of this paper. This research has made use of the Simbad database operated at CDS, Strasbourg, France, and was supported by the Swiss National Science Foundation.
© European Southern Observatory (ESO) 1999
Online publication: December 16, 1998