Astron. Astrophys. 324, 617-623 (1997)

Acknowledgements

This work is supported by the Fonds zur Förderung der wissenschaftlichen Forschung (FWF), projects S7305-AST, S7308-AST and J01283-AST. We thank the referee Dr. M.A.T. Groenewegen for valuable comments on the manuscript.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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