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Astron. Astrophys. 321, 444-451 (1997) AcknowledgementsI am grateful to Juan Pérez-Mercader for his guidance and help, and also to Massimo Persic and Paolo Salucci for their hospitality at SISSA, where a part of this work has been done, and for their helpful comments. I would also like to thank the referee for his help in improving this paper. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1997 Online publication: June 30, 1998 ![]() |