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Astron. Astrophys. 354, 1086-1090 (2000) AcknowledgementsWe are grateful to the referee for his careful review of the article and for his helpful comments. Part of this work was supported by ASI Grant. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 2000 Online publication: February 25, 2000 ![]() |