Measurement of Ds+ to mu+ nu and the Decay constant f_Ds

We examine e+e- to Ds- Ds*+ or Ds-* Ds+ collisions at 4170 MeV using the CLEO-c detector in order to measure the decay constant f_Ds+. We use the Ds+ to ell+ nu channel, where the ell+ designates either a mu+ or a tau+. Analyzing both modes simultaneously, we determine B(D_s^+ to mu+ nu)= (0.657 +- 0.090 +- 0.028)%, B(D_s^+ to tau+ nu)= (7.1 +- 1.4 +- 0.3)%, and extract f_Ds+ = 282 +- 16 +- 7 MeV. Combining with our previous determination of B(D+ to mu+ nu), we find that the ratio f_Ds+/f_D+ = 1.27 +- 0.12 +- 0.03. (All new results here are preliminary.) We compare with current theoretical estimates.