Test of Lepton Quantum Numbers by Muon Capture

It is shown that the $\Delta Q=2\mathrm{}$ semileptonic process can be tested by experiments on muonic atoms. The $\Delta$ resonances in nuclear ground states will lead to a ${\mu }^{-}\to e^{+}$ reaction presently observable if the $\Delta Q=2\mathrm{}$, $\Delta S=0\mathrm{}$ interaction is within about two orders of magnitude of the ordinary strangeness-conserving interactions. The consequences for various lepton schemes is discussed.