Previous measurements of the ratio of semileptonic decays $R(D^{*})=B(B \to D^{*} \tau^+ \nu) / B(B \to D^{*} \mu^+ \nu)$ and $R(D^0)=B(B \to D^0 \tau^+ \nu) / B(B \to D^0 \mu^+ \nu)$ from Babar, Belle, and LHCb hint at a deviation from lepton universality. This seminar presents the first joint measurement of both $R(D^*)$ and $R(D^0)$ at LHCb, reconstructed with the $\tau^+ \to \mu^+ \nu \bar{\nu}$ decay mode in the $D^{*+} \mu$ and $D^0 \mu$ final states and assuming isospin symmetry. This pathfinder analysis is based on Run 1 data. This result supersedes the previous LHCb measurement of $R(D^{(*+)})$ alone with the muonic decay of the tau.
Jan Fiete Grosse-Oetringhaus, Michelangelo Mangano, Monica Pepe-Altarelli and Pedro Silva