  We use a QCD relativistic potential model to compute the strong coupling
constant $g$ appearing in the effective Lagrangian which describes the
interaction of $0^-$ and $1^-$ $\bar q Q$ states with soft pions in the limit
$m_Q \to \infty$. We compare our results with other approaches; in particular,
in the non relativistic limit, we are able to reproduce the constituent quark
model result: $g=1$, while the inclusion of relativistic effects due to the
light quark gives $g={1 \over 3}$, in agreement with QCD sum rules. We also
estimate heavy meson radiative decay rates, with results in agreement with
available experimental data.
