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Astron. Astrophys. 347, 419-423 (1999) AcknowledgementsI am grateful to the anonymous referee for his/her suggestions, I thank Prof. R.G. Lin (CfA, Guangzhou Normal University, China) for his help with the data and Dr. Y. Copin (CRAL, Observatoire de Lyon, France) for his help with the figures. This work is supported by the National Natural Scientific Foundation of China and the National Pandeng Project of China.
© European Southern Observatory (ESO) 1999 Online publication: June 30, 1999 ![]() |