We are grateful to the anonymous referee for helpful arguments and comments. This work is partially supported by the RFBR grant 98-02-16801 and the program `Universitety Rossii' (grant 5559) of the Ministry of Teaching and Professional Education, Russia. GVL is thankful to RFBR project `Molodye uchenye Rossii' of 1999.
© European Southern Observatory (ESO) 2000
Online publication: March 28, 2000