  We discuss and clarify the validity of effective single field theories of
inflation obtained by integrating out heavy degrees of freedom in the regime
where adiabatic perturbations propagate with a suppressed speed of sound. We
show by construction that it is indeed possible to have inflationary
backgrounds where the speed of sound remains suppressed and slow-roll persists
for long enough. In this class of models, heavy fields influence the evolution
of adiabatic modes in a manner that is consistent with decoupling of physical
low and high energy degrees of freedom. We emphasize the distinction between
the effective masses of the isocurvature modes and the eigenfrequencies of the
propagating high energy modes. Crucially, we find that the mass gap that
defines the high frequency modes increases with the strength of the turn, even
as the naive heavy (isocurvature) and light (curvature) modes become more
strongly coupled. Adiabaticity is preserved throughout, and the derived
effective field theory remains in the weakly coupled regime, satisfying all
current observational constraints on the resulting primordial power spectrum.
In addition, these models allow for an observably large equilateral
non-Gaussianity.
