Measurement of B(Ds+→l+ν) and the decay constant fDS+

We examine e(+)e(-) -> Ds-Ds*+ and Ds*-Ds+ interactions at 4170 MeV using the CLEO-c detector in order to measure the decay constant fD(s)(+). We use the D-s(+) -> l(+)nu channel, where the l(+) designates either a mu(+) or a tau(+), when the tau(+) -> pi(+), when the tau(+) -> pi(+)(v) over bar .Analyzing both modes independently, we determine B(D-s(+) -> mu(+)nu)=(0.594 +/- 0.066 +/- 0.031)%, and B(D-s(+) ->tau(+)nu)=(8.0 +/- 1.3 +/- 0.4)%. We also analyze them simultaneously to find an effective value of Beff(D-s(+) ->mu(+)nu)=(0.638 +/- 0.059 +/- 0.033)% and extract fD(s)(+)=274 +/- 13 +/- 7 MeV. Combining with our previous determination of B(D+ -> mu(+)nu), we also find the ratio fD(s)(+)/fD(+)=1.23 +/- 0.11 +/- 0.04. We compare to current theoretical estimates. Finally, we find B(D-s(+) -> e(+)nu)< 1.3X10(-4) at 90% confidence level.