Subleading corrections to the vertical bar Vub vertical bar determination from exclusive B decays

It has been proposed to determine the CKM matrix element vertical bar Vub vertical bar 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→ρeν. We estimate the leading corrections to this symmetry relation, of order αs(mb) and Λ/mb, 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 vertical bar Vub vertical bar of the order of a few percent