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Astron. Astrophys. 342, 687-703 (1999) AcknowledgementsWe are grateful to the referee, Prof. Gopal Krishna, for his precious comments and really careful reading of the manuscript. We feel that his suggestions and criticisms have certainly improved this paper. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1999 Online publication: February 23, 1999 ![]() |