Improved Measurement of cal{B}(D^+ --> mu^+ nu) and the Pseudoscalar   Decay Constant f_{D^+}
  We extract a relatively precise value for the decay constant of the D+ meson by measuring B(D+ -> mu+ nu)=(4.40 +/- 0.66 {+0.09}{-0.12}) x 10^{-4}, using 281 inverse-pb of data taken on the psi(3770) resonance with the CLEO-c detector. We find fD+=(222.6 +/- 16.7 {+2.8}{-3.4}) MeV, and compare with current theoretical calculations. We also set a 90% confidence upper limit on B(D+ -> e+ nu)<2.4 x 10^{-5} which limits contributions from non-standard model physics. 
M. Artuso, et al, CLEO Collaboration