We use chiral perturbation theory to evaluate the scattering amplitude for the process Pi^+ K^- to Pi^+ K^- at leading and next-to-leading orders in the chiral counting and in the presence of isospin breaking effects. We also discuss the influence of the latter on the combination of the S-wave Pi K scattering lengths which is relevant for the 2S - 2P energy levels shift of K Pi atoms.
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