Authors thank W.D. Watson, Yu.A. Schekinov, R.P. Norris, J.L. Caswell and C.M. Walmsley for useful discussions, and an anonymous referee for suggestions which improved the paper. D.M.C. and P.D.G. thank the Australian Research Council for financial support of this work. A.M.S. thanks the Russian Foundation for Fundamental Research for financial support (grant 96-02-19689) and is grateful to the University of Sydney Research Centre for Theoretical Astrophysics for the opportunity to visit Australia, which also helped in improving this paper.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998