The research was supported by the EU-HCM programme, contract number CHRX-CT94-0445; A. Manara, A. Cellino, M. Di Martino and V. Zappalà were also partially supported by grant ASI-94-RS-69 of Italian Space Agency (ASI). The authors would like to thank also M.E. Bailey, P. Farinella and A. Morbidelli for useful discussions and suggestions and Bill Napier for improving the English.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998