We wish to thank A. Garcia for help in the drawing of figures and A. Mekkas for technical assistance. We are grateful to the anonymous referee for a critical reading of the manuscript and constructive comments. This research has made use of the services of the Centre de Données Stellaires, Strasbourg, France. Special thanks to Judy Coughlin for a most valuable discussion.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 2000