Measurements of $\mathcal R(D^*)$ at LHCb

Lepton universality violation would represent a signal of physics be-yond the Standard Model. Semileptonic decays of beauty mesons in third-generationleptons are particularly interesting, since anomalies have been observed in the mea-surement of their Branching Ratios. This contribution reports on the measurementsof the observable $\mathcal{R(D^∗)=B(B ^\prime →D^{∗−} \tau +ντ)/B(B ^\prime→D^{∗−}μ+νμ)}$ performed atLHCb using both leptonic and hadronic decay channels of theτlepton.