The authors thank SAO/NASA ADS Astronomy Abstract Service for their deposit authorization. The original publication in the Proceedings of the 31st Liege International Astrophysical Colloquium (LIAC 93), held June 21-25, 1993, Institut d'Astrophysique, Liege. Edited by J. Surdej, D. Fraipont-Caro, E. Gosset, S. Refsdal, and M. Remy. Liege: Universite de Liege, Institut d'Astrophysique.
All documents in ORBi are protected by a user license.