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Astron. Astrophys. 318, 621-630 (1997) AcknowledgementsThe author thanks Jean Heyvaerts for fruitful discussions and careful reading of the manuscript. The numerical calculations were performed on the CRAY C98 at I.D.R.I.S., Orsay (France). ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1997 Online publication: July 8, 1998 ![]() |