Astron. Astrophys. 319, 525-534 (1997)

Acknowledgements

We would like to thank R. Waters and A. Treves for useful discussions. TB is supported by NWO under grant PGS 78-277; SC gratefully acknowledges the receipt of an Italian Space Agency fellowship. LZ was supported in part by NSF grant AST 93-15133 and NASA grant 5-2925 at the University of Illinois.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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