We thank Dr. E. Bajaja for reading the manuscript constructively. We are grateful to the members of the computational staff Messrs. F.A. Bareilles, C. Cristina Miguel and M. Fumagalli, as well as Mrs. N. Perón for their helpful computational assistance. We thank the helpful comments by the referee, Prof. A. Blaauw.
© European Southern Observatory (ESO) 2000
Online publication: June 26, 2000