Radiative leptonic decays of heavy mesons

We compute the photon spectrum and the rate for the decays B(D)→lνl γ. These photonic modes constitute a potentially large background for the purely leptonic decays which are used to extract the heavy meson decay constants. While the rate for D→lνγ is small, the radiative decay in the B meson case could be of comparable magnitude or even larger than B→μν. This would affect the determination of fB if the τ channel cannot be identified. We obtain theoretical estimates for the photonic rates and discuss their possible experimental implications