Subleading shape functions in B → Xul(υ)over-bar and the determination of |Vub|

We calculate the subleading twist contributions to the endpoint of the inclusive lepton energy spectrum in B --> X(u)l (nu) over bar. We show that the same two subleading twist functions that appear in the decay B --> X(s)gamma govern the subleading effects in this decay. Using these results we find large O(Lambda(QCD)/m(B)) corrections to the determination of \V-ub\ from the endpoint of the charged lepton spectrum. Using a simple model for the relevant subleading shape functions, we estimate the uncertainty in \V-ub\ from Lambda(QCD)/m(b) corrections to be at the similar to15% level for a lower lepton energy cut of 2.2 GeV. (C) 2002 Elsevier Science B.V. All rights reserved.