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Astron. Astrophys. 347, 272-276 (1999) AcknowledgementsWe are grateful to C. Charbonnel for instructive discussions. This work was supported by Deutsche Forschungsgemeinschaft under grant SFB 375-95. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1999 Online publication: June 18, 1999 ![]() |