Astron. Astrophys. 357, 669-676 (2000)

Acknowledgements

We are indebted to an anonymous referee for useful suggestions. It is a pleasure to thank Dr. Uli Ott for his careful and useful check of the manuscript. This work was partly supported by the MURST Italian grant Cofin98. ML gratefully acknowledges the support of a OPRS grant.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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