  The density perturbations generated when the inflaton decay rate is perturbed
by a light scalar field $\chi$ are studied. By explicitly solving the
perturbation equations for the system of two scalar fields and radiation, we
show that even in low energy-scale inflation nearly scale-invariant spectra of
scalar perturbations with an amplitude set by observations are obtained through
the conversion of $\chi$ fluctuations into adiabatic density perturbations. We
demonstrate that the spectra depend on the average decay rate of the inflaton &
on the inflaton fluctuations. We then apply this new mechanism to string
cosmologies & generalized Einstein theories and discuss the conditions under
which scale-invariant spectra are possible.
