Subleading corrections to the |V_ub| determination from exclusive B decays

It has been proposed to determine the CKM matrix element |V_ub| in a model-independent way from a combination of rare and semileptonic B and D decays near the zero recoil point. An essential ingredient in such a determination is a heavy quark symmetry relation connecting the form-factors appearing in B → K*e^-e^- to semileptonic form factors relevant for B → ρev. We estimate the leading corrections to this symmetry relation, of order α_s(m_b) and Λ/m_b, pointing out that they can be as large as 20%, depending on the value of the matrix element of a dimension-4 operator. Dimensional analysis estimates of this matrix element give a corresponding uncertainty in |Vu_b| of the order of a few percent.