Soft-Photon Corrections to B[over ¯]→Dτ^{-}ν[over ¯]_{τ} Relative to B[over ¯]→Dμ^{-}ν[over ¯]_{μ}.

We evaluate long-distance electromagnetic (QED) contributions to (B) over bar (0) -> D+ tau-(v) over bar (tau) and B- -> D-0 tau(-)(v)over bar>(tau) relative to (B) over bar (0) -> D+ mu(-)(v) over bar (mu) and B- -> D-0 mu(-)(v) over bar (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 similar to 4% and similar to 3%, respectively, for the soft-photon energy cut in the range 20-40 MeV.