  We discuss the possibility to measure in present experiments, especially
LHCb, the non leptonic decay branching ratio $B \to D' \pi$, and emphasize
phenomenological implications on $B \to D' l \nu$ semileptonic decay. We have
estimated by lattice QCD the $D'$ decay constant $f_{D'}$ that parameterizes
the $D'$ emission contribution to the Class-III non leptonic decay $B^- \to D^0
\pi^-$. In addition, we provide a new estimate of the decay constants
$f_{D_{s,q}}$ which read $f_{D_{s}}=252(3)$ MeV and
$f_{D_{s}}/f_{D}=1.23(1)(1)$.
