  Recent measurements of exclusive $B^{-} \to \tau^{-}\,\nu$ and $B^0 \to
\pi^{+}\,l^{-}\,\bar{\nu}_l$ decays via the $b \to u\,l\,\nu$ transition
process differ from the standard model expectation and, if they persist in
future $B$ experiments, will be a definite hint of the physics beyond the
standard model. Similar hints of new physics have been observed in $b \to c$
semileptonic transition processes as well. BABAR measures the ratio of
branching fractions of $B \to (D,\,D^{\ast})\,\tau\,\nu$ to the corresponding
$B \to (D,\,D^{\ast})\,l\nu$, where $l$ represents either an electron or a
muon, and finds $3.4\sigma$ discrepancy with the standard model expectation. In
this context, we consider a most general effective Lagrangian for the $b \to
u\,l\,\nu$ and $b \to c\,l\,\nu$ transition processes in the presence of new
physics and perform a combined analysis of all the $b \to u$ and $b \to c$
semi-(leptonic) data to explore various new physics operators and their
couplings. We consider various new physics scenarios and give predictions for
the $B_c \to \tau\nu$ and $B \to \pi\tau\nu$ decay branching fractions. We also
study the effect of these new physics parameters on the ratio of the branching
ratios of $B \to \pi\tau\nu$ to the corresponding $B \to \pi\,l\,\nu$ decays.
