We want to thank the staff of the ESO La Silla observatory for their support during our observations. We would also like to thank T. Lanz for very helpful discussions and the referees J. Cohen and B. Behr for useful remarks. S.M. acknowledges financial support from the DARA under grant 50 OR 96029-ZA. A.V.S. acknowledges financial support from NASA Astrophysics Theory Program proposal NRA-99-01-ATP-039.
© European Southern Observatory (ESO) 2000
Online publication: July 27, 2000