  The branching fraction ratios of $\overline{B}^0\to
D^+\tau^-\overline{\nu}_{\tau}$ and $\overline{B}^0\to
D^{*+}\tau^-\overline{\nu}_{\tau}$ decays are measured with respect to their
muonic counterparts, using a data sample corresponding to an integrated
luminosity of 2.0 fb$^{-1}$ collected by the LHCb experiment in proton-proton
collisions at $\sqrt{s} = 13$ TeV. The reconstructed final states are formed by
combining $D^+$ mesons with $\tau^-\to\mu^-\overline{\nu}_{\mu}\nu_{\tau}$
candidates, where the $D^+$ is reconstructed via the $D^+\to K^-\pi^+\pi^+$
decay. The results are
  \begin{align*}
  R(D^{+}) &= 0.249 \pm 0.043 \pm 0.047,
  R(D^{*+}) &= 0.402 \pm 0.081\pm 0.085,
  \end{align*} where the first uncertainties are statistical and the second
systematic. The two measurements have a correlation coefficient of $-0.39$ and
are compatible with the Standard Model.
