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Astron. Astrophys. 333, 970-976 (1998) AcknowledgementsWe would like to thank the referee of the paper for his constructive criticism and his cooperation and Franca Chiuderi Drago for her fruitful comments. Maria Massi acknowledges support for her research by the European Union under contract FMGECT950012. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: April 28, 1998 ![]() |