  Semileptonic $b \to c$ transitions, and in particular the ratios $R(D^{(*)})
= \frac{\Gamma(B \rightarrow D^{(*)} \tau \nu )}{\Gamma(B \rightarrow D^{(*)}
\ell \nu )}$, can be used to test the universality of the weak interactions. In
light of the recent discrepancies between the experimental measurements of
these observables by BaBar, Belle and LHCb and the Standard Model predicted
values, we study the robustness of the latter. Our analysis reveals that $R(D)$
might be enhanced by lepton mass effects associated to the mostly unknown
scalar form factor. In constrast, the Standard Model prediction for $R(D^*)$ is
found to be more robust, since possible pollutions from $B^*$ contributions
turn out to be negligibly small, which indicates that $R(D^*)$ is a promising
observable for searches of new physics.
