  We perform a calculation of the strong interaction effects between the $B$
and $D$ mesons in the $\bar{B} \to D \bar{\nu} \ell$ reaction, as a crossing
process of reactions with $BD$ in the final state, where the strong interaction
between the mesons leads to a bound $BD$ state. We find corrections to the tree
level amplitude of the order of $15-25\%$. We further see the effect of the
corrections studied in the $R_D$ ratio for the rates of $\bar{B} \to D
\bar{\nu}_\tau \tau $ and $\bar{B} \to D \bar{\nu}_e \ell$ decays and find
corrections of the order of $10\%$. Given the claims of $1.5\%$ precision in
this ratio from fits to data within the standard model, any theoretical model
aiming at describing this ratio within the same precision must take into
account the corrections described in the present work.
