Watson's theorem and electromagnetism in K -> pi pi decay

We consider what constraints unitarity and CPT invariance yield on the stro ng and electromagnetic phases entering K --> pi pi 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 pi (+)pi (-)gamma channel. This demonstrate s that Watson's theorem can be extended to include the presence of electrom agnetism. We point out the consequences for the general structure of the K --> pi pi decay amplitudes in the presence of isospin violation. (C) 2001 E lsevier Science B.V. All rights reserved.