  Heavy fields coupled to the inflaton reduce the speed of sound in the
effective theory of the adiabatic mode each time the background inflationary
trajectory deviates from a geodesic. This can result in features in the
primordial spectra. We compute the corresponding bispectrum and show that if a
varying speed of sound induces features in the power spectrum, the change in
the bispectrum is given by a simple formula involving the change in the power
spectrum and its derivatives. In this manner, we provide a uniquely
discriminable signature of a varying sound speed for the adiabatic mode during
inflation that indicates the influence of heavy fields. We find that features
in the bispectrum peak in the equilateral limit and, in particular, in the
squeezed limit we find considerable enhancement entirely consistent with the
single field consistency relation. From the perspective of the underlying
effective theory, our results generalize to a wide variety of inflationary
models where features are sourced by the time variation of background
quantities. A positive detection of such correlated features would be
unambiguous proof of the inflaton's nature as a single light scalar degree of
freedom embedded in a theory that is UV completable.
