We thank Dr. Álvaro Giménez, Dr. Antonio Claret, Dr. Georges Meynet and Dr. Daniel Schaerer for their useful suggestions. This work has been supported by CICYT under contract PB95-0185 and by the Acciones Integradas Hispano-Francesas (HF95/328B). R.A. acknowledges financial support from the Ministerio de Educación y Ciencia.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998