We acknowledge support from a Royal Society/Chinese Academy of Sciences Joint Project. ZZ acknowledges support from Chinese National Natural Science Foundation. Major parts of this work have been completed while ZZ was visiting the MPA Garching, funded by the Max-Planck-Gesellschaft. We thank Andrew King for improving the language of the manuscript and an anonymous referee for helpful comments.
© European Southern Observatory (ESO) 2000
Online publication: August 23, 2000