We would like to thank the referee for his helpful comments. This work was supported by the Deutsche Forschungsgemeinschaft through the grant LE 1039/2-1, 3-1 and the NATO Scientific Collaboration Grants program. GTB also gratefully acknowledges support by the AvH foundation through a Fedor Lynen fellowship, in the course of which substantial parts of this contribution were developed. PPK acknowledges support from the Natural Science and Engineering Research Council of Canada, and of a Killam Fellowship.
© European Southern Observatory (ESO) 2000
Online publication: December 8, 1999