  Both the light hypernucleus ${}_\Lambda {\rm H}^4$ and the nucleus ${\rm
He}^4$ have spin 0. The ratio $R$ of the electron to muon rates for the pure
Fermi transitions \begin{equation} R=\frac{\Gamma\left({}_\Lambda {\rm
H}^4\rightarrow {\rm e}^- + \bar \nu +{\rm He}^4\right)}
{\Gamma\left({}_\Lambda {\rm H}^4\rightarrow {\mu}^- + \bar \nu +{\rm
He}^4\right)} \end{equation} is a sensitive measure of the presence of a {\em
second-class} weak current (to the extent that SU(3) is valid in strong
interactions), and hence, is a test of the Standard Model. Rates and
sensitivities, using Da$\Phi$ne, the $e^+e^-$ machine under construction at
Frascati, are discussed.
