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Astron. Astrophys. 354, 815-822 (2000) AcknowledgementsThis project is partly supported by the Chinese National Natural Foundation. I thank Dr. S. Mao, Dr. H. J. Mo and Prof. S. D. M. White for detailed discussions, and the useful help of anonymous referee. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 2000 Online publication: February 25, 2000 ![]() |