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Astron. Astrophys. 324, 770-777 (1997) AcknowledgementsWe are grateful to the reviewer B.A.S. Gustafson for many valuable comments and discussions. This work was supported in part by the Italian Space Agency (ASI). ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1997 Online publication: May 26, 1998 ![]() |