  We have carried out a search for Main Belt Asteroids (MBAs) co-orbiting with
the large MBA Vesta and the dwarf planet Ceres. Through improving the search
criteria used in (Christou, 2000b) and numerical integrations of candidate
coorbitals, we have identified approximately 51 (44) objects currently in
co-orbital libration with Ceres (Vesta). We show that these form part of a
larger population of transient coorbitals; 129 (94) MBAs undergo episodes of
co-orbital libration with Ceres (Vesta) within a 2 Myr interval centred on the
present. The lifetime in the resonance is typically a few times 10^5 yr butcan
exceed 2*10^6 yr. The variational properties of the orbits of several
co-orbitals were examined. It was found that their present states with respect
to the secondary are well determined but knowledge of it is lost typically
after ~2*10^5 years. Objects initially deeper into the coorbital region
maintain their coorbital state for longer. Using the model of Namouni et al.
(1999) we show that their dynamics are similar to those of temporary coorbital
NEAs of the Earth and Venus. As in that case, the lifetime of resonant
libration is dictated by planetary secular perturbations, the inherent
chaoticity of the orbits and close encounters with massive objects other than
the secondary. In particular we present evidence that, while in the coorbital
state, close encounters with the secondary are generally avoided and that Ceres
affects the stability of tadpole librators of Vesta. Finally we demonstrate the
existence of Quasi-satellite orbiters of both Ceres and Vesta and conclude that
decametre-sized objects detected in the vicinity of Vesta by the DAWN mission
may, in fact, belong to this dynamical class rather than be bona-fide (i.e.
keplerian) satellites of Vesta.
