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Published online by Cambridge University Press: 22 December 2003
It was recently shown in N-body simulations that the simpleadiabatic invariant r M(r) describes the evolution of acontracting spherical density distribution very well. This is surprising as this adiabatic invariant is only valid forcircular orbits. Orbits in spherically symmetric potentialsresemble rather precessing ellipses. We want to highlight someproblems involved in proving the validity of the adiabaticapproximation on a orbit to orbit basis. Statistical methods aremore promising.