Global well-posedness for the KP-II equation on the background of a non localized solution

Motivated by transverse stability issues, we address the time evolution under the KP-II flow of perturbations of a solution which does not decay in all directions, for instance the KdV-line soliton. We study two different types of perturbations : perturbations that are square integrable in $\R\times \T$ and perturbations that are square integrable in $\R^2$. In both cases we prove the global well-posedness of the Cauchy problem associated with such initial data.