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Astron. Astrophys. 331, 934-948 (1998) AcknowledgementsWe would like to thank the referee for constructive comments that have led to improvements of this paper. This work was supported by the Swiss National Science Foundation. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: March 3, 1998 ![]() |