  We study the secular evolution of several exoplanetary systems by extending
the Laplace-Lagrange theory to order two in the masses. Using an expansion of
the Hamiltonian in the Poincar\'e canonical variables, we determine the
fundamental frequencies of the motion and compute analytically the long-term
evolution of the keplerian elements. Our study clearly shows that, for systems
close to a mean-motion resonance, the second order approximation describes
their secular evolution more accurately than the usually adopted first order
one. Moreover, this approach takes into account the influence of the mean
anomalies on the secular dynamics. Finally, we set up a simple criterion that
is useful to discriminate between three different categories of planetary
systems: (i) {\it secular} systems (HD 11964, HD 74156, HD 134987, HD 163607,
HD 12661 and HD 147018); (ii) systems {\it near a mean-motion resonance} (HD
11506, HD 177830, HD 9446, HD 169830 and $\upsilon$ Andromedae); (iii) systems
{\it really close to} or {\it in a mean-motion resonance} (HD 108874, HD 128311
and HD 183263).
