We wish to thank the referee, J. Solf, for his useful comments. MC would like to acknowledge funding from Fobairt, the Irish Science and Technology Agency. Finally we would like to express our gratitude to the staff of the La Palma Observatory for their assistance. The Isaac Newton Telescope on the island of La Palma is operated by the Royal Greenwich Observatory at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. This research has made use of the Simbad database, operated at the CDS, Strasbourg, France.
© European Southern Observatory (ESO) 1998
Online publication: July 20, 1998