Measurement of the ratio of branching fractions B(${\mathit{D}}^0$→${\mathrm{\pi }}^{\mathrm{-}}$${\mathit{e}}^{+}$${\nu }_{\mathit{e}}$)/B(${\mathit{D}}^0$→${\mathit{K}}^{\mathrm{-}}$${\mathit{e}}^{+}$${\nu }_{\mathit{e}}$)

Using 3.0 ${\mathrm{fb}}^{\mathrm{-}1}$ of data collected with the CLEO-II detector, we study the Cabibbo-suppressed decay ${\mathit{D}}^0$→${\mathrm{\pi }}^{\mathrm{-}}$${\mathit{e}}^{+}$${\nu }_{\mathit{e}}$. The ratio of the branching fractions B(${\mathit{D}}^0$→${\mathrm{\pi }}^{\mathrm{-}}$${\mathit{e}}^{+}$${\nu }_{\mathit{e}}$)/B(${\mathit{D}}^0$→${\mathit{K}}^{\mathrm{-}}$${\mathit{e}}^{+}$${\nu }_{\mathit{e}}$) is measured to be (10.3±3.9±1.3)%, corresponding to an upper limit of 15.6% at the 90% confidence level.