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Astron. Astrophys. 364, 232-236 (2000) AcknowledgementsWe wish to thank the referee, J.M.Moran, for a critical reading of the manuscript and very useful comments. We thank D.R.Jiang for valuable discussions. Xu is grateful to the staff of the Laboratory for Astronomical Data Analysis and Computational Physics in Nanjing university for their assistance during his visit. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 2000 Online publication: December 15, 2000 ![]() |