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Astron. Astrophys. 356, 873-887 (2000) AcknowledgementsWe would like to thank the referee S. G. Ryan for helpful and motivating discussions and for providing us with some of the observational data. This work was supported by the Swiss Nationalfonds. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 2000 Online publication: April 17, 2000 ![]() |