One author (C.F.) would like to express his sincere gratitude to the CNRS for supporting his stay and Paris Observatory for kind hospitality. This work was partly supported by a key project from the National Science Committee and the National Natural Science Foundation of P.R.China. Another author (J.C.H.) thanks also CNRS and NSFC for support and Nanjing university for kind hospitality. Both authors are pleased to thank a unknown Referee for useful suggestions.
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998