  It has been commonly assumed that pseudoscalar contributions to the leptonic
decay of charged mesons, like pions and kaons, is strongly constrained due to
the helicity suppression present in the ratio $R_{l/l'} = \Gamma(P \rightarrow
l\nu[\gamma])/\Gamma(P \rightarrow l'\nu[\gamma])$, where $P$ are the charged
pseudoscalar meson and $l,l' = e,\mu,\tau$. Here we show that if the effective
couplings are proportional to the corresponding charged lepton masses (and also
the PMNS matrix), the constraints from $R_{l/l'}$ are entirely avoided, and a
rather new large allowed region is permitted in the parameter space. In the
case of the electron, we found a non-trivial region in the range
$10^{-4}\lesssim (G^\eta/G_F) \lesssim 10^{-3}$, where $G^\eta$ is the
effective pseudoscalar coupling associated with a novel charged scalar field,
$\eta$, and $G_F$ is the Fermi constant. Furthermore, we show that this
dependence of the pseudoscalar couplings on the charged lepton masses can
naturally be associated with a critical class beyond the standard model
physics, namely models without (leptonic) flavor-changing neutral currents in
the scalar sector. The most known examples are the models that satisfy the
so-called Glashow-Weinberg-Paschos theorem. Finally, we also point out that, in
those cases, the decay rate is degenerated with the Standard Model prediction,
possibly hiding the new physics effects in those decays.
