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Astron. Astrophys. 355, 713-719 (2000) AcknowledgementsWe thank an anonymous referee for useful suggestions. The work of RLMC and AM has been supported by a grant of the Spanish DGES PB97-1435-C02-01. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 2000 Online publication: March 9, 2000 ![]() |