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Astron. Astrophys. 351, 368-372 (1999) AcknowledgementsThe author would like to thank an anonymous referee for his/her constructive comments that helped to improve the paper significantly. This work was supported in part by a grant from the Panden Project and a grant from the National Natural Science Foundation of China. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1999 Online publication: November 2, 1999 ![]() |