LD and HVW acknowledge support from the Science Foundation of Flanders. EJB acknowledges support form the National Science Foundation (Grant No. AST-9315124). This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France and the Vienna Atomic Line Data-base (VALD), Vienna, Austria. The authors thank the anonymous referee for his interesting comments.
© European Southern Observatory (ESO) 1998
Online publication: March 30, 1998