The authors gratefully acknowledge the support of M.B. Cita who made possible this study. We are indebted to R. von Grafenstein, R. Zahn, N. Combourieu Nebout, and P. Belanger for many fruitful discussion and the use of their data. We also thank W. Landuzzi and G. Rovatti for laboratory assistance. C. Laj and L. Sagnotti helped to improve the manuscript by valuable comments. This is IGM publication No. 1100.