It is a pleasure to thank Dr. Debang Lao and Dr. Guo Yang for supplying us with IDL software for analyzing the data; we also thank the staff of the BAO for providing help to complete this paper. This work is supported by National Nature Science Foundation project No. 19773016 and 19833050. We also appreciate Dr. M.Poquerusse spending plenty of time to improve this paper.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001