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Astron. Astrophys. 360, 345-350 (2000) AcknowledgementsThe numerical calculations were performed on the CRAY C98 at I.D.R.I.S., Orsay (France). The author thanks Jean Heyvaerts for fruitful discussions, and the anonymous referee for fruitful comments. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 2000 Online publication: July 27, 2000 ![]() |