It is a pleasure to thank Dr. M. Marconi for providing the data of Fig. 5 in computer form and for many helpful suggestions and discussions. We also wish to thank Dr. A. Alonso, Dr. S. Gentili, and Dr. F. D'alessio for their collaboration during the observations. Special thanks go to Dr. G. Bono and Dr. E. Brocato for many useful discussions and to our referee, Jan Lub, for carefully reviewing the paper and for helpful suggestions.
© European Southern Observatory (ESO) 1998
Online publication: April 28, 1998