This work was partially carried out while S.V. Chalov during extended stays in 1997 was a guest at the Institute for Astrophysics and Extraterrestrial Research of the University of Bonn. The authors are grateful to the Deutsche Forschungsgemeinschaft (DFG) for the financial support of these stays in the frame of a bi-national cooperation project with grant number: 436 RUS113/110/2. S.V.Chalov was also partially supported by the Russian Foundation for Basic Research (RFBR) Grant 95-02-042-15.
© European Southern Observatory (ESO) 1998
Online publication: June 18, 1998