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Astron. Astrophys. 335, 1-11 (1998) AcknowledgementsWe would like to thank Peter Schneider for several comments and suggestions that have helped us improve the paper. This work has been partially supported by MURST and by ASI of Italy. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: June 12, 1998 ![]() |