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Astron. Astrophys. 328, 130-142 (1997) AcknowledgementsThis work was supported in part by the Netherlands Organization for Scientific Research (NWO) under grant PGS 78-277. SPZ thanks the Institute for Advanced Study for the hospitality extended during his visits. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1997 Online publication: March 24, 1998 ![]() |