We evaluate long-distance electromagnetic (QED) contributions toBrelative toB 0 → D þ μ −ν μ and B − → D 0 μ −ν μ , respectively, in the standard model. We point out that the QED corrections to the ratios RðD þ Þ and RðD 0 Þ 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 longdistance QED corrections depend on the masses and relative velocities of the daughter particles. We find that theoretical predictions for RðD þ Þ τ=μ and RðD 0 Þ τ=μ can be amplified by ∼4% and ∼3%, respectively, for the soft-photon energy cut in the range 20-40 MeV. DOI: 10.1103/PhysRevLett.120.261804 The semileptonic B meson decays that are at the elementary level induced by the b → clν l transitions provide a potentially interesting avenue for testing the standard model (SM) at low energies. In this respect, it turns out useful to construct the ratios RðHÞ, H ¼ D, D Ã between the branching fractions that involve τ leptons and those involving light leptons. These observables do not depend on the Cabibbo-Kobayashi-Maskawa matrix element V cb and are also theoretically cleaner due to the (partial) cancellation of the hadronic uncertainties parametrized by the corresponding form factors. The forthcoming Belle II experiment is expected to reduce the corresponding measurement uncertainties to the level of around 3% [1], comparable to the current theoretical uncertainties. This is also the typical size of electromagnetic (QED) effects, which we turn to study in this Letter, focusing on long-distance QED effects in RðDÞ.Short-distance electroweak (EW) contributions to branching fractions of semileptonic decays were evaluated to 1.3% [2-4], but since such corrections are lepton universal, they cancel in the ratio RðDÞ. The complete understanding of QED effects in meson decays is a nontrivial task due to the complicated interplay with QCD dynamics, e.g., structure-dependent contributions that probe the hadronic content [5][6][7]. In this Letter, we evaluate the lepton-massdependent soft-photon effects, which give rise to important corrections.We point out terms that distinguish the cases of the neutral and charged B decaysThe up-to-date average [2] of the lattice-QCD predictions [8,9] iswhich is consistent with previous evaluations involving different approaches (see [10][11][12][13][14] which combines electrons and muons for the decay into the light lepton and averages neutral and charged B decays. The averaged experimental result exceeds the SM expectation at the level of 2.2σ. Combined with current discrepancy with respect to the SM in RðD Ã Þ, these have been considered as a hint of physics beyond the SM.One should note that the measured results partially include soft photons using PHOTOS Monte Carlo generator [19,20] for the simulation of modifications of the kinematic variables induced by final-state photon radiations [21][22][23].