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Astron. Astrophys. 329, 845-852 (1998) AcknowledgementsWe thank an anonymous referee for helpful criticisms of an earlier version of this paper. This work is partially supported by the Brazilian research agency CNPq. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: December 16, 1997 ![]() |