Measurement of the decay constant fDs+ using Ds+→l+ν

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