  We evaluate long-distance electromagnetic (QED) contributions to $\bar{B}{}^0
\to D^+ \tau^{-} \bar{\nu}_{\tau}$ and $B^- \to D^0 \tau^{-} \bar{\nu}_{\tau}$
relative to $\bar{B}{}^0 \to D^+ \mu^{-} \bar{\nu}_{\mu}$ and $B^- \to D^0
\mu^{-} \bar{\nu}_{\mu}$, respectively, in the standard model. We point out
that the QED corrections to the ratios $R(D^{+})$ and $R(D^{0})$ are not
negligible, contrary to the expectation that radiative corrections are almost
canceled out in the ratio of the two branching fractions. The reason is that
long-distance QED corrections depend on the masses and relative velocities of
the daughter particles. We find that theoretical predictions for
$R(D^{+})^{\tau/\mu}$ and $R(D^{0})^{\tau/\mu}$ can be amplified by $\sim4\%$
and $\sim3\%$, respectively, for the soft-photon energy cut in range $20$-$40$
MeV.
