Semileptonic decays B→ρlν and D→ρlν and the heavy-quark symmetry

We determine the first correction in inverse powers of heavy-quark masses to the ratio of the differential decay rates of B→ρeν and D→ρeν at the kinematical point of zero hadronic recoil (maximum q2) within the quark model. In this particular case of heavy-to-light meson transitions, the heavy-quark limit gives a definite prediction, and theoretical uncertainties should be minimal. This result is potentially important for the extraction of the Kobayashi-Maskawa element Vub from future measurements.