Chiral perturbation theory with virtual photons and leptons

We construct a low-energy effective field theory which allows for a full tr eatment of the isospin-breaking effects in semileptonic weak interactions. To this end, we enlarge the particle spectrum of chiral perturbation theory with virtual photons by including also the light leptons as dynamical degr ees of freedom. Using super-heat-kernel techniques, we determine the additi onal one-loop divergences generated by the presence of virtual leptons and give the full list of associated local counterterms. We illustrate the use of our effective theory by applying it to the decays pi --> l(nu l) and K - -> l(nu l).