The purely leptonic decays D-s --> iotav and D-s --> muv are studied in a sample of four million hadronic Z decays collected with the ALEPH detector at the LEP e(+)e(-) collider from 1991 to 1995. The branching fractions are extracted from a combination of two analyses, one optimized to select D-s --> iotav decays with iota --> ev (v) over bar or muv (v) over bar, and the other optimized for D-s --> muv decays. The results are used to evaluate the D-s decay constant, within the Standard Model: f(Ds) = 285 +/- 19(stat) +/- 40(syst) MeV. (C) 2002 Elsevier Science B.V. All rights reserved. References: 26
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