Discussions with the Hipparcos group at the University of Barcelona, in particular Jordi Torra, Francesca Figueras, David Fernández, and Montserrat Mestres, contributed to shape the work presented here. I am pleased to thank the careful reading and helpful comments by Prof. Per Olof Lindblad and by the referee, Prof. Adriaan Blaauw.
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999