Measurement of $D_{s}^{-} \to \tau^{-}\overline{\nu}_\tau$ and a New Limit for $B^- \to \tau^{- }\overline{\nu}_\tau$

Using a data sample of 1,475,000 $\ZQQG$ events collected during 1994 with the L3 detector at LEP, we have studied the purely leptonic decays of heavy flavour mesons, $\Dtau$ and $\Btau$. A signal is observed in the invariant mass distribution $\MDgamma$ corresponding to the decay sequence $\Dstar \rightarrow \gamma {\rm D^{-}_{s}}$, ~$\Dtau$, ~$\tauto \lnunu$. The branching fraction for $\Dtau$ decays is measured to be {$\BR (\Dtau) = 0.074 \pm 0.028(stat) \pm 0.016(syst) \pm 0.018(norm)$}. No signal of $\Btau$ decays is observed in the data, corresponding to an upper limit on the branching fraction $\BR(\Btau) < 5.7 \times 10^{-4}$ at 90\% CL. \end{abstract}