We consider the system Sun-Jupiter-Ceres as an example of a planar, ci
rcular, restricted three-body problem and, after substituting the mass
ratio of Jupiter/Sun (which is approximately 10(-3)) with a parameter
epsilon, we prove the existence of stable quasiperiodic motions with
frequencies close to the observed (average) frequencies reported in ''
The Astronomical Almanac'' for \epsilon\ < 10(-6). The proof is ''comp
uter-assisted''.