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Astron. Astrophys. 327, 947-951 (1997) AcknowledgementsThis work is supported by an RGC grant of Hong Kong Government, a CRCG grant of the University of Hong Kong, and the National Natural Scientific Foundation of China. The authors thank the anonymous referee for his constructive comments and T.C. Boyce for the critical reading. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1997 Online publication: April 6, 1998 ![]() |