The work was supported by the RFFI (95-02-6053) and the INTAS (93-3364) grants. The work of S.P. was also supported by the ISSEP. We thank Dr. I.E. Panchenko, the unknown referee and Dr. J. Lequeux, who made a lot of suggestions to improve the article (especially the quality of the language) and Prof. S.R. Pottasch for his help.
© European Southern Observatory (ESO) 1998
Online publication: February 16, 1998