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Astron. Astrophys. 342, 1-14 (1999)
Acknowledgements
We thank the referee for his careful reading of the manuscript and his helpful suggestions. This work has been partially supported through NATO Grant CRG 920150, EEC Contract ERB-CHRX-CT92-0033, CNR Contract 95.01099.CT02 and by Institut National des Sciences de l'Univers and Cosmology GDR.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998
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