We study the non equilibrium time evolution of an integrable field theory in
1+1 dimensions after a sudden variation of a global parameter of the
Hamiltonian. For a class of quenches defined in the text, we compute the long
times limit of the one point function of a local operator as a series of form
factors. Even if some subtleties force us to handle this result with care,
there is a strong evidence that for long times the expectation value of any
local operator can be described by a generalized Gibbs ensemble with a
different effective temperature for each eigenmode
