hints of lepton flavor universality in tree-level (charged-current) $B$-decays, which are independent of the above discussion since $h_{A_1}(1) |V_{cb}|$ cancel out in these observables. The experimental averages of the LFUV ratios $R_{D^{(\ast)}}$ are given by [@Amhis:2016xyh] $$\begin{aligned}
R_D^{\mathrm{exp}}= \dfrac{\mathcal{B}(B\to D\tau \bar{\nu})}{\mathcal{B}(B\to D l \bar{\nu})}=0.41(5)\,,\qquad\quad R_{D^\ast}^{\mathrm{exp}}= \dfrac{\mathcal{B}(B\to D^{\ast}\tau \bar{\nu})}{\mathcal{B}(B\to D^{\ast}l \bar{\nu})}=0.304(15)\,,
\label{RD_RDstar_exp}\end{aligned}$$ to be compared with the SM predictions $R_D^{\mathrm{SM}}=0.299(7)$ [@Na:2015kha; @Lattice:2015rga] and $R_{D^\ast}^{\mathrm{SM}}=0.257(3)$ [@Bernlochner:2017jka], which are respectively $\approx 2\,\sigma$ and $\approx 3\,\sigma$ larger than the experimental values given above. [^5] As anticipated in the introduction, the main advantage of using ratios as in the equations above is that many hadronic uncertainties actually cancel, providing theoretically clean observables. Nonetheless, there are still residual hadronic uncertainties that must be carefully estimated. An important ingredient in the computation of $R_{D^{(\ast)}}$ is that the $\tau$-lepton mass is not negligible in comparison to $m_B$. Therefore, the (pseudo)scalar form factors, which do not contribute to the rates with light leptons, are needed to reliably predict these quantities. In the case of $R_D$ both scalar, $f_0(q^2)$, and vector, $f_+(q^2)$, form factors have been computed on the lattice in the region of large $q^2$ by two different collaborations [@Na:2015kha; @Lattice:2015rga]. Extrapolation to $q^2=0$ is highly constrained by the 