On the mixings of heavy neutrinos and muon decays

We set limits on the mixing of heavy neutrinos into the muon family, ${\left|U_{\mu i}\right|}^2$, for neutrino masses in the range 30 to 70 MeV/${\mathit{c}}^2$. Both the Michel parameter and the $e^{+}$ spectrum are used. This complements limits from leptonic decays of pseudoscalar mesons. We also give indications of how current experiments can be used to improve these limits.