GB acknowledges support by SFB 375. YPJ would like to thank the Alexander-von-Humboldt Foundation for a Research Fellowship. He also thanks Max-Planck Institute für Astrophysik for financial support during the final stage of this work. We thank the referee, F.R. Bouchet, for insightful comments.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998