\mu \bar{\nu})}=0.71 \pm 0.17 \pm 0.18\,,$$ which turns out to be again $\approx 2\sigma$ larger than the SM estimate, $R_{J/\psi}^{\mathrm{SM}}=0.23(1)$, obtained by combining LQCD and QCD sum rules results [@RJpsipaper]. These independent measurements can be useful not only to corroborate/refute the excess found on $R_{D^{(\ast)}}$, but also to provide a complementary information about the underlying NP structure. For instance, as discussed in Sec. \[sec:eft\], contributions in the form of (current)$\times$(current) operators predict the following equality, $$\begin{aligned}
\dfrac{R_D}{R_D^{\mathrm{SM}}}=\dfrac{R_{D^\ast}}{R_{D^\ast}^{\mathrm{SM}}}=\dfrac{R_{J/\psi}}{R_{J/\psi}^{\mathrm{SM}}} = \dots\,,\end{aligned}$$ for all decay modes based on the same transition. On the other hand, scalar and/or tensor operators induce a distinct correlation between $P\to P^{\prime}$ and $P\to V$ decays, where $P^{(\prime)}$ and $V$ denote generic pseudoscalar and vector mesons, respectively. Another possibility proposed in the literature is to consider the angular observables of the decay $B\to D^\ast(\to D \pi) \tau \bar{\nu}$, which have different sensitivities to the operators in Eq.  [@Becirevic:2016hea; @Ivanov:2017mrj]. Note, in particular, that a first measurement of the $\tau$-lepton polarization asymmetry in $B\to D^\ast \tau \bar{\nu}$ has been made by Belle, with large error bars [@Hirose:2016wfn]. This observable is particularly sensitive to pseudoscalar and tensor contributions, but it is unaffected by interaction of the type $(V-A) \times (V-A)$ [@Azatov:2018knx].

