  In multi-field inflation one or more non-adiabatic modes may become light,
potentially inducing large levels of isocurvature perturbations in the cosmic
microwave background. If in addition these light modes are coupled to the
adiabatic mode, they influence its evolution on super horizon scales. Here we
consider the case in which a non-adiabatic mode becomes approximately massless
("ultralight") while still coupled to the adiabatic mode, a typical situation
that arises with pseudo-Nambu-Goldstone bosons or moduli. This ultralight mode
freezes on super-horizon scales and acts as a constant source for the curvature
perturbation, making it grow linearly in time and effectively suppressing the
isocurvature component. We identify a Stuckelberg-like emergent shift symmetry
that underlies this behavior. As inflation lasts for many e-folds, the
integrated effect of this source enhances the power spectrum of the adiabatic
mode, while keeping the non-adiabatic spectrum approximately untouched. In this
case, towards the end of inflation all the fluctuations, adiabatic and
non-adiabatic, are dominated by a single degree of freedom.
