SEMILEPTONIC DECAYS B-]RHO-L-NU AND D-]RHO-L-NU AND THE HEAVY-QUARK SYMMETRY

We determine the first correction in inverse powers of heavy-quark mas ses to the ratio of the differential decay rates of B-->rhoenu and D-- >rhoenu at the kinematical point of zero hadronic recoil (maximum q2) within the quark model. In this particular case of heavy-to-light meso n transitions, the heavy-quark limit gives a definite prediction, and theoretical uncertainties should be minimal. This result is potentiall y important for the extraction of the Kobayashi-Maskawa element V(ub) from future measurements.