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Astron. Astrophys. 338, 777-780 (1998) AcknowledgementsI thank X. P. Wu for helpful discussions and an anonymous referee for valuable comments and suggestions. This work was supported by the National Natural Science Foundation of China. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: September 17, 1998 ![]() |