  The universality of the weak interactions can be tested in semileptonic $b
\to c$ transitions, and in particular in the ratios $R(D^{(*)}) \equiv \Gamma(B
\rightarrow D^{(*)} \tau \nu )/\Gamma(B \rightarrow D^{(*)} \ell \nu )$ (where
$\ell = \mu$ or $e$). Due to the recent differences between the experimental
measurements of these observables by BaBar, Belle and LHCb on the one hand and
the Standard Model predicted values on the other hand, we study the predicted
ratios $R(D^{(*)}) = \Gamma(B \rightarrow D^{(*)} \tau +$ "missing" $)/\Gamma(B
\rightarrow D^{(*)} \ell \nu )$ in scenarios with an additional sterile heavy
neutrino of mass $\sim 1$ GeV. Further, we evaluate the newly defined ratio
$R(0) \equiv \Gamma(B \to \tau +$ "missing" $)/\Gamma(B \to \mu \nu )$ in such
scenarios, in view of the future possibilities of measuring the quantity at
Belle-II.
