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Astron. Astrophys. 363, 415-424 (2000) AcknowledgementsWe thank Andreas Obrist, Sabine Schindler and Yoel Rephaeli for interesting discussions. We are grateful to the referee for useful comments. This work has been supported by the Dr Tomalla Foundation and by the Swiss National Science Foundation.
© European Southern Observatory (ESO) 2000 Online publication: December 11, 2000 ![]() |