  We perform a dynamical lattice computation of the Isgur-Wise functions
tau_{1/2} and tau_{3/2} at zero recoil. We consider three different light quark
masses corresponding to 300 MeV < m_{PS} < 450 MeV, which allow us to
extrapolate our results to the physical u/d quark mass. We find tau_{1/2}(1) =
0.296(26) and tau_{3/2}(1) = 0.526(23). Uraltsev's sum rule is saturated up to
80% by the ground state. We discuss implications regarding semileptonic decays
B --> X_c l nu and the associated "1/2 versus 3/2" puzzle.
