Isospin violation in low-energy charged pion–kaon scattering

We evaluate the isospin breaking corrections to the scattering amplitude π−K+→π−K+ at threshold in the framework of chiral perturbation theory. This channel is of particular interest for the strong 2S–2P energy level shift in pion–kaon bound states. While a prediction of this level shift is hampered by a large uncertainty in the isoscalar scattering length, we find only a moderate uncertainty of about 3% in the electromagnetic corrections which are relevant for the extraction of the scattering lengths from experiment.