  The measurement of $R_D$ ($R_{D^*}$), the ratio of the branching fraction of
$\overline{B} \to D \tau \bar{\nu}_\tau (\overline{B} \to D^* \tau
\bar{\nu}_\tau)$ to that of $\overline{B} \to D l \bar{\nu}_l (\overline{B} \to
D^* l \bar{\nu}_l)$, shows $1.9 \sigma$ $(3.3 \sigma)$ deviation from its
Standard Model (SM) prediction. The combined deviation is at the level of $4
\sigma$ according to the Heavy Flavour Averaging Group (HFAG). We perform an
effective field theory analysis (at the dimension 6 level) of these potential
New Physics (NP) signals assuming $ SU(3)_{C} \times SU(2)_{L} \times U(1)_{Y}$
gauge invariance. We first show that, in general, $R_D$ and $R_{D^*}$ are
theoretically independent observables and hence, their theoretical predictions
are not correlated. We identify the operators that can explain the experimental
measurements of $R_D$ and $R_{D^*}$ individually and also together. Motivated
by the recent measurement of the $\tau$ polarisation in $\overline{B} \to D^*
\tau \bar{\nu}_\tau$ decay, $P_\tau^{D^*}$ by the {\sc Belle} collaboration, we
study the impact of a more precise measurement of $P_\tau^{D^*}$ (and a
measurement of $P_\tau^D$) on the various possible NP explanations.
Furthermore, we show that the measurement of $R_{D^*}$ in bins of $q^2$, the
square of the invariant mass of the lepton neutrino system, along with the
information on $\tau$ polarisation, can completely distinguish the various
operator structures.
