Measurements of the exclusive branching fractions B($D^0$→${\pi }^{\mathrm{-}}$$e^{+}$${\nu }_e$) and B($D^0$→$K^{\mathrm{-}}$$e^{+}$${\nu }_e$), using data collected at the ψ(3770) with the Mark III detector at the SLAC $e^{+}$$e^{\mathrm{-}}$ storage ring SPEAR, are used to determine the ratio of the Kobayashi-Maskawa matrix elements ‖$V_{\mathrm{cd}}$/$V_{\mathrm{cs}}$${\Vert }^2$ =0.057${\pm }_{0.015}^{+0.038\mathrm{}}$±0.005.

• Received 30 December 1988

DOI:https://doi.org/10.1103/PhysRevLett.62.1821