We are grateful to the Chinese 2.16m Telescope time allocation committee for their support of this programme and to the staff and telescope operators of the Xinglong Station of Beijing Astronomical Observatory for their support. Especially we would like to thank Prof. J.Y. Hu and Dr. J.Y. Wei for their active cooperation which enabled all of the observations to go through smoothly. We are also deeply grateful to Henrique R. Schmitt for kindly providing us the procedure of stellar population synthesis. We also thank the anonymous referee for helpful comments and constructive suggestions. Special thanks to Dr. S. Mao and Dr. T. Kiang for their hard work of English revision of this paper. This work was supported by grants from the National Panden Project and Natural Science Foundation of China.
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999