The work of A. Raga was supported by the CONACyT grant E9608-3166P. A. Raga acknowledges the support of the Observatoire de Paris for an "Enseignant Invité" position, which funded a visit to the Observatoire during which part of this work was carried out. We thank Matt Redman (the referee) for helpful comments.
© European Southern Observatory (ESO) 1999
Online publication: May 6, 1999