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Astron. Astrophys. 355, 818-828 (2000) AcknowledgementsWe are grateful to J. Heyvaerts for stimulating discussions. S.A. acknowledges support from the Deutsche Forschungsgemeinschaft (SFB 359), and T.L. was partly supported by an operating grant from NSERC of Canada. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 2000 Online publication: March 9, 2000 ![]() |