We would like to express our thanks to the referee, Dr. Pourbaix, for helpful advices. This research has made use of the Simbad data base, operated at CDS, Strasbourg, France. This work has been performed using the computing facilities provided by the OCA program "Simulations Interactives et Visualisation en Astronomie et Mécanique (SIVAM)".
© European Southern Observatory (ESO) 2000
Online publication: December 11, 2000