We have studied the leptonic decay D + s → τ + ν τ , via the decay channel τ + → e + ν eντ , using a sample of tagged D + s decays collected near the D * ± s D ∓ s peak production energy in e + e − collisions with the CLEO-c detector. We obtain B(D + s → τ + ν τ ) = (5.30 ± 0.47 ± 0.22)% and determine the decay constant f Ds = (252.5 ± 11.1 ± 5.2) MeV, where the first uncertainties are statistical and the second are systematic. * Deceased

Analyzing 600 pb À1 of e þ e À collisions at 4170 MeV center-of-mass energy with the CLEO-c detector, we measure the branching fraction BðD þ s ! þ Þ ¼ ð5:52 AE 0:57 AE 0:21Þ% using the þ ! þ " decay mode. Combining with other CLEO measurements of BðD þ s ! þ Þ we determine the pseudoscalar decay constant f D s ¼ ð259:7 AE 7:8 AE 3:4Þ MeV consistent with the value obtained from our D þ s ! þ measurement of ð257:6 AE 10:3 AE 4:3Þ MeV. Combining these measurements we find a value of f D s ¼ ð259:0 AE 6:2 AE 3:0Þ MeV, that differs from the most accurate prediction based on unquenched lattice gauge theory of ð241 AE 3Þ MeV by 2.4 standard deviations. We also present the first measurements of BðD þ s ! K 0 þ 0 Þ ¼ ð1:00 AE 0:18 AE 0:04Þ%, and BðD þ s ! þ 0 0 Þ ¼ ð0:65 AE 0:13 AE 0:03Þ%, and measure a new value for BðD þ s ! þ Þ ¼ ð8:9 AE 0:6 AE 0:5Þ%.

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