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

It has been proposed to determine the CKM matrix element \Vub\ 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 --> rhoev. We estimate the leading corrections to this symmetry relation, of order alpha(s) (m(b)) and A/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 \V-ub\ of the order of a few percent. (C) 2002 Published by Elsevier Science B.V.