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Astron. Astrophys. 327, 145-154 (1997) AcknowledgementsWe thank the anonymous referee for useful comments and suggestions. This work is supported by INTAS grant 93-2478 and ISF grants UCT000 and UCT200. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1997 Online publication: April 8, 1998 ![]() |