  We construct a low-energy effective field theory which allows the full
treatment of 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 degrees of
freedom. Using super-heat-kernel techniques, we determine the additional
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}.
