Measurement of the semileptonic branching fractions of the D0 meson

Using the ARGUS detector at the e + e − storage ring DORIS II, we have measured the semileptonic branching ratios of the D 0 meson to be Br ( D 0 → e + ν e X ) = (6.9 ± 0.3 ± 0.5) % and Br ( D 0 → μ + ν μ X ) = (6.0 ± 0.7 ± 1.2) %, and the semileptonic branching ratio of the charmed hadron mixture from e + e − annihilation around 10 GeV to be Br ( c → e + ν e X ) = (9.6 ± 0.4 ± 1.1) %.