We gratefully acknowledge the many people whose kind cooperation allowed the completion of this work. In particular, we thank our anonymous referee for his stringent criticisms helping us to be less optimistic in approaching the problem of VLM structures. We are deeply indebted with F.Allard for providing us with the new color-temperature relation for solar metallicity, with G.Chabrier for providing his zero metal EOS, with C.C. Dahn for making available his data for dwarfs and, last but not least, with Ivan King, G. Piotto and A. Cool for allowing the use of the splendid HST data for NGC6397. S.C. warmly thanks O. Straniero and J. Fernandes for stimulating discussion on this issue.
© European Southern Observatory (ESO) 1997