We would like to express our thanks to M.J. Goupil and Y. Lebreton for their assistance in handling the CESAM code, J.-P. Zahn for clarifying discussions and the referee for helpful advice about a first version of the article. Y.-C. Lo acknowledges financial support from the "Ministère de l'enseignement supérieur et de la recherche".
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998