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Astron. Astrophys. 327, 758-770 (1997) AcknowledgementsI am indebted to two anonymous referees for their careful examination of both theoretical and observational aspects. Their comments significantly improved the paper. This work was supported by the Deutsche Forschungsgemeinschaft through grants FI 681/2-1, FI 681/1-1. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1997 Online publication: April 6, 1998 ![]() |