We thank the referee, F. Palla, for his valuable and convenient comments and suggestions. The computations presented in this paper were performed at the "Centre de Calcul Intensif de l'Observatoire de Grenoble" and at "Informatique Mathématiques Appliquées Grenoble" on a IBM SP1 computer. These facilities are financed by the MESR, CNRS, and Région Rhône-Alpes.
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998