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Astron. Astrophys. 345, 439-447 (1999) AcknowledgementsH.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 ![]() |