  From sum rules in the heavy quark limit of QCD, using the non-forward
amplitude, we demonstrate that if the slope rho^2 = -xsi'(1) of the Isgur-Wise
function xsi(w) attains its lower bound 3/4 (as happens in the BPS limit
proposed by Uraltsev), the IW function is completely determined, given by the
function xsi(w) = [2/(w+1)]^(3/2).
