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Astron. Astrophys. 355, L43-L46 (2000) AcknowledgementsWe thank R. Wijers for his valuable comments. This work was partly supported by the National Natural Science Foundation of China, grants 19773007 and 19825109, and the National Climbing Project on Fundamental Researches.
© European Southern Observatory (ESO) 2000 Online publication: March 21, 2000 ![]() |