Author is grateful to Prof. Yu.D.Kotov for support of this work and to Dr. Yu.E.Lyubarsky for fruitful discussions. Gratefuls follow also to unknown referee and to Prof. K Tsinganos (Univ. of Crete) for useful remarks and large work on improvement of the manuscript. This work was partially supported by the Russian Fund of Basic Research (RFBR) N96-02-17113-a.
© European Southern Observatory (ESO) 1997
Online publication: April 6, 1998