Watson's theorem and electromagnetism in Κ → ππ decay

We consider what constraints unitarity and CPT invariance yield on the strong and electromagnetic phases entering Κ → ππ decay. In particular, we show that the relative size of the electromagnetically-induced changes in the I = 0 and I = 2 phase shifts in the two-pion final state do not depend on the explicit coupling to the π⁺π^-γ channel. This demonstrates that Watson's theorem can be extended to include the presence of electromagnetism. We point out the consequences for the general structure of the Κ → ππ decay amplitudes in the presence of isospin violation.