H.A. is grateful to the Spanish Ministry of Foreign Affairs for a MUTIS Studentship. J.P. acknowledges financial support from the Spanish DGICYT project PB96-0921. Dr. Joshua Barnes is thanked for making available his TreeCode , and L. Verdes-Monten egro and A. Del Olmo for their comments which helped to improve the presentation of the paper. The Editor and an anonymous referee are also thanked for helpful comments.
© European Southern Observatory (ESO) 1999
Online publication: April 19, 1999