  We consider the dynamics of a small trojan companion of a hypothetical giant
exoplanet under the secular perturbations of additional planets. By a suitable
choice of action-angle variables, the problem is amenable to the study of the
slow modulation, induced by secular perturbations, to the dynamics of an
otherwise called `basic' Hamiltonian model of two degrees of freedom (planar
case). We present this Hamiltonian decomposition, which implies that the slow
chaotic diffusion at resonances is best described by the paradigm of
modulational diffusion.
