We are indebted to the referee, P. Reig, for his extremely useful comments and suggestions. We are also grateful to Drs. J. Yan and Y.-N. Fu of Purple Mountain Observatory for their valuable discussion and to Drs. J.-Y. Wei and X.-J. Jiang of Beijing Astronomy Observatory for their assistance in observations. QZL acknowledges the financial support in observations from United Laboratory of Optical Astronomy, Academia Sinica. This research was partially supported by the national Climbing Programme on fundamental researches.
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999