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Astron. Astrophys. 347, 391-400 (1999) AcknowledgementsThis work was supported by the European Commission under the TMR Programme, contract number FMRX-CT98-0168. The authors thank Prof A. Achterberg for fruitful discussions. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1999 Online publication: June 18, 1999 ![]() |