Radiative semileptonic $$ \overline{B} $$ decays

We consider the form factors for radiative semileptonic decays $\bar{B}(v) \rightarrow D^{(*)}(v') \ell {\bar \nu}_\ell \gamma$ in kinematic region where photon momentum, $k$, is small enough that heavy quark symmetry (HQS) can be applied without radiated changing velocity (i.e., $v^{(\prime)} \cdot k < m_{(b,c)}$). find HQS remarkably powerful, leaving only four new undetermined at leading order $1/m_{(b,c)}$. In addition, one of them fixed terms Isgur-Wise function region, $v^{(\prime)}\cdot \Lambda_\text{QCD}$.