A Measurement of the Rate of Charm Production in W Decays
  Using data recorded at centre-of-mass energies around 183 GeV and 189 GeV with the OPAL detector at LEP, the fundamental coupling of the charm quark to the W boson has been studied. The ratio Rc^W = Gamma(W -> cX)/Gamma(W -> hadrons) has been measured from jet properties, lifetime information, and leptons produced in charm decays. A value compatible with the Standard Model expectation of 0.5 is obtained: Rc^W = 0.481 +- 0.042(stat.) +- 0.032(syst.). By combining this result with measurements of the W boson total width and hadronic branching ratio, the magnitude of the CKM matrix element Vcs is determined to be |Vcs| = 0.969 +- 0.058. 
The OPAL collaboration, G. Abbiendi, et al