  The ratios of branching fractions
$\mathcal{R}(D^{*})\equiv\mathcal{B}(\bar{B}\to
D^{*}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(\bar{B}\to
D^{*}\mu^{-}\bar{\nu}_{\mu})$ and $\mathcal{R}(D^{0})\equiv\mathcal{B}(B^{-}\to
D^{0}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(B^{-}\to
D^{0}\mu^{-}\bar{\nu}_{\mu})$ are measured, assuming isospin symmetry, using a
sample of proton-proton collision data corresponding to 3.0 fb${ }^{-1}$ of
integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The
tau lepton is identified in the decay mode
$\tau^{-}\to\mu^{-}\nu_{\tau}\bar{\nu}_{\mu}$. The measured values are
$\mathcal{R}(D^{*})=0.281\pm0.018\pm0.024$ and
$\mathcal{R}(D^{0})=0.441\pm0.060\pm0.066$, where the first uncertainty is
statistical and the second is systematic. The correlation between these
measurements is $\rho=-0.43$. Results are consistent with the current average
of these quantities and are at a combined 1.9 standard deviations from the
predictions based on lepton flavor universality in the Standard Model.
