Astron. Astrophys. 323, 395-398 (1997)

Acknowledgements

We sincerely thank Mr. Bing Zhang, Mr. Zheng Zheng and Mr. Jifeng Liu for helpful discussions. We are very grateful to Dr. J.M. Rankin for her careful reading the manuscript. This work is supported by the National Natural Sciences Foundation of China and by Doctoral Programme Foundation of Institution of Higher Education in China. GJQ thanks support from Climbing Projects - the National key project for fundamental research of China. JLH acknowledges support from the Research Foundation for Young Scientists of the Chinese Academy of Sciences (CAS), the Su-Shu Huang Astrophysics Research Foundation of CAS, and the Astronomical Research Foundation of CAS.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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