JR-G would like to express his thanks to the Deutsche Forschungsgemeinschaft (DFG) for its financial support during part of this work. GT-T gratefully acknowledges the Banco Bilbao Vizcaya foundation for a visiting professorship at the University of Cambridge. We also would like to thank Dr. T. Mahoney for a careful reading of the manuscript.
© European Southern Observatory (ESO) 1998
Online publication: February 4, 1998