Measurement of the Decay Constant $f_D{}_S^{+}$ using $D_S^+ --> ell^+ nu

We measure the decay constant fDs using the Ds -> l+ nu channel, where the l+ designates either a mu+ or a tau+, when the tau+ -> pi+ nu. Using both measurements we find fDs = 274 ±13 ± 7 MeV. Combining with our previous determination of fD+, we compute the ratio fDs/fD+ = 1.23 ± 0.11 ± 0.04. We compare with theoretical estimates.