$B^- \to \mu^- {\bar \nu}_\mu \gamma$ and the Determination of $f_B$
  We compute the rate of the decay $B^- \to \mu^- {\bar \nu}_\mu \gamma$ by a QCD relativistic potential model, with the result: ${\cal B}(B^- \to \mu^- {\bar \nu}_\mu \gamma)=0.9 \cdot 10^{-6}$. We also discuss how this decay mode can be used to access $f_B$. 
F. De Fazio