This work has been supported by the Deutsche Forschungsgemeinschaft, DFG (grants Scho 394/13 and La 587/16). I greatly appreciate previous improvements to the evolution code and stimulating support by T. Blöcker. Further more I would like to thank R. Gallino, L. Koesterke, N. Langer and D. Schönberner for advice, helpful discussions and proof reading. The referee, A. Bressan, is acknowledged for his very useful comments.
© European Southern Observatory (ESO) 2000
Online publication: August 23, 2000