We are greatly indebted to our referee for several helpful comments. We thank V. Castellani and G. Bono for discussion and critical suggestions. Financial support for this work was provided by the Ministero dell'Università e della Ricerca Scientifica e Tecnologica (MURST) under the scientific project "Stellar Evolution" (Vittorio Castellani, coordinator).
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000