We want to express our appreciation to D. Pfenniger for many discussions and suggestions. We are indepted to G. Contopoulos and D. Merritt for their comments on an early version of this work. Discussions with L. Athanassoula and T. de Zeeuw were indeed very beneficial. Special thanks are addressed to J. Cresson for his remarks regarding the topological instability and the diffusion of orbits through transition chains in systems with 3 or more degrees of freedom. Finally, we are deeply grateful to P. Robutel for his continuous help in the course of this research. This work was partially supported by a grant of the French ministry of research and education (MRE), by the CEE contract CHRX-CT94-0460 and by the CNRS.
© European Southern Observatory (ESO) 1998
Online publication: December 8, 1997