Non-decoupling and lepton number violation in left-right models

We argue that large non-decoupling effects of heavy neutrinos can appear naturally in manifestly left-right symmetric models due to the minimization conditions of the scalar potential and the structure of vev's imposed by phenomenology. We derive constraints on off-diagonal light-heavy and heavy-heavy neutrino mixings from the searches for lepton violating decays $\mu\rightarrow e\gamma,$ $\mu\rightarrow e e^-e^+$ and $\mu-e$ conversion in nuclei. The most stringent limits come from the latter process because its amplitude shows a quadratic non-decoupling dependence on the heavy neutrino mass. Due to the suppression of right-handed currents by large $W_R$ mass the present experiments are not sensitive to the intergenerational mixings between heavy neutrinos if $M_{W_R}>200$ TeV.