The author is grateful to the referee Dr. T. Lloyd Evans for valuable comments. This work was supported by APART (Austrian Programme for Advanced Research and Technology) from the Austrian Academy of Sciences, the Österreichische Nationalbank under the Jubiläumsfonds-project number 6876 and the Fonds zur Förderung der wissenschaftlichen Forschung under project number S7308-AST. We used the Simbad database operated at CDS, Strasbourg, France.
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999