Effects of non-standard couplings, radiative corrections and neutrino masses on the lepton spectrum in $\mu$ and $\tau$ decays

We investigate the combined effect of neutrino masses and non-standard couplings on the various lepton spectra in $\mu$ and $\tau$ decays. We emphasize the energy spectra of the neutrinos, which can be measured in the new KARMEN experiment and hence will yield novel information on deviations from the $V-A$ structure. To constrain these couplings, QED radiative corrections have to be taken into account. We evaluated them and found small corrections to the neutrino spectra.