We would especially like to thank Prof. V.B. Baranov for having stimulated this research and for useful discussions. Many thanks also to Dr. S.V. Chalov for the fruitful discussions we had with him. We acknowledge the support of the International Science Foundation and the Russian Goverment under contract N J2E100 and by the Russian Foundation of Fundamental Investigations under contract N 95-02-042-15. We thank our referee D. Rucinski for his very careful and competent analysis of the paper, which led to the detection of an error in the comparison with the classical model, and his numerous useful comments.
© European Southern Observatory (ESO) 1997