We are grateful to S. Charlot, G. Bruzual, M. Dantel-Fort, G. Mathez, E. Hatziminaoglou, J.P. Kneib, Y. Mellier, D. Maccagni, D. Valls-Gabaud and J.-P. Picat for interesting comments and discussion. This work has been done within the framework of the VIRMOS collaboration. We would thank L. Moscardini and E. Carretta for useful suggestions on the preliminary version of the manuscript. Part of this work was supported by the French Centre National de la Recherche Scientifique , by the French Programme National de Cosmologie (PNC), and the TMR Lensnet ERBFMRXCT97-0172 (http://www.ast.cam.ac.uk/IoA/lensnet ). J.-M.M. thanks the JSPS Postdoctoral Fellowship Grant (P98176) for financial support.
© European Southern Observatory (ESO) 2000
Online publication: December 11, 2000