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Astron. Astrophys. 340, 617-625 (1998) AcknowledgementsI am very grateful to Dr. K. Werner, the referee, for his comments and corrections to the original version of this paper. This research has been supported by grant No. 2 P03D 013 14 from the Polish Committee for Scientific Research. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: November 9, 1998 ![]() |