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Astron. Astrophys. 342, 34-40 (1999) AcknowledgementsWe are grateful to E. Recami and E. Spedicato for helpful and stimulating discussions during the period in which this work was performed and the referee Prof. P. Laguna for some useful comments and suggestions. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1999 Online publication: December 22, 1998 ![]() |