We present a quenched lattice study of the vector form factors at zero-momentum transfer, f(V)(0), relevant for the determination of |V(us)| from semileptonic K -> pi l nu (K(l3)) and Sigma(-) -> nl nu decays. Using suitable double ratios of three-point correlation functions we show that in case of both kaon and hyperon decays it is possible to calculate the form factor f(V)(0) at the percent precision, which is the one required for a significant determination of |V(us)|. In case of K(l3) decay the leading quenched chiral logarithms are corrected for by using analytic calculations in quenched chiral perturbation theory. Our final result, f(V) (K(0)pi(-)) = 0.960 +/- 0.005(stat)+/- 0.007(syst), where the systematic error does not include the residual quenched effects, is in good agreement with the estimate made by Leutwyler and Roos. The impact of our result on the extraction of |V(us)| is briefly illustrated.
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