A dispersive treatment of

The Kℓ4 decay is one of the best sources of information on some low-energy constants of chiral perturbation theory (χPT). We present a dispersive approach to Kℓ4 decays, which takes rescattering effects fully into account. The dispersion relation treats both experimentally accessible form factors simultaneously and also describes the dependence on the dilepton invariant mass. We apply isospin-breaking corrections before fitting the data of NA48/2 and E865 experiments and extract the values of the low-energy constants L1r, L2r and L3r from a matching to χPT