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Astron. Astrophys. 347, 194-202 (1999) AcknowledgementsFinancial support from the Swedish Natural Science Research Council (NFR) is acknowledged. We are grateful to the referee, Dr. J. Alcolea, for valuable comments on the manuscript. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1999 Online publication: June 18, 1999 ![]() |