  The BaBar measurements of the ratios ${\cal R}(D^{(*)})=\frac{{\cal B}(B \to
D^{(*)} \tau {\bar \nu}_\tau)}{{\cal B}(B \to D^{(*)} \mu {\bar \nu}_\mu)}$
deviate from the standard model expectation, while new results on the purely
leptonic $B \to \tau {\bar \nu}_\tau$ mode show a better consistency with the
standard model, within the uncertainties. In a new physics scenario, one
possibility to accomodate these two experimental facts consists in considering
an additional tensor operator in the effective weak hamiltonian. We study the
effects of such an operator in a set of observables, in semileptonic $B \to
D^{(*)}$ modes as well as in semileptonic $B$ and $B_s$ decays to excited
positive parity charmed mesons.
