  The prediction of a nearly scale-invariant spectrum of curvature and tensor
fluctuations is among the main features of cosmic inflation. The current
measurements of the primordial fluctuations in the cosmic microwave background
(CMB) provide tight constraints on the amplitude of the scalar and tensor
spectra, and the scalar tilt. However, the precise connection between these
observables and a given inflationary model, depends on the expansion history
between the end of inflation and the beginning of the radiation dominated era,
which corresponds to the reheating epoch. This mapping between horizon exit and
reentry of fluctuations, parametrized by the number of $e$-folds $N_*$, can
therefore be affected by the presence of a transient epoch of non-perturbative
particle production during reheating ({\em preheating}). Using a combination of
perturbative and lattice computations, we quantify the impact of preheating in
a non-equilibrated dark matter sector on the CMB observables, under the
assumption of a simultaneous perturbative decay of the inflaton into Standard
Model particles. Combined with structure formation constraints, this allows us
to impose stringent bounds on the post-inflationary reheating temperature.
