Chiral Perturbation Theory for $B \rightarrow D^*$ and $B \rightarrow D$   Semileptonic Transition Matrix Elements at Zero Recoil
  Heavy quark symmetry predicts the value of $B \rightarrow D$ and $B \rightarrow D^*$ transition matrix elements of the current $\bar c \gamma_\mu (1 - \gamma_5)b$, at zero recoil (where in the rest frame of the $B$ the $D$ or $D^*$ is also at rest). We use chiral perturbation theory to compute the leading corrections to these predictions which are generated at low momentum, below the chiral symmetry breaking scale. 
L. Randall and M. B. Wise