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

We consider the form factors for the radiative semileptonic decays $$ \overline{B} $$ (v) → D(*)(v′)ℓ $$ \overline{\nu} $$ ℓγ in the kinematic region where the photon momentum, k, is small enough that heavy quark symmetry (HQS) can be applied without the radiated photon changing the heavy quark velocity (i.e., v(′) ∙ k < m(b,c)). We find that HQS is remarkably powerful, leaving only four new undetermined form factors at leading order in 1/m(b,c). In addition, one of them is fixed in terms of the leading order Isgur-Wise function in the kinematic region, v(′) ∙ k < ΛQCD.