  We investigate the problem of perturbative reheating and its effects on the
evolution of the curvature perturbations in tachyonic inflationary models. We
derive the equations governing the evolution of the scalar perturbations for a
system consisting of a tachyon and a perfect fluid. Assuming the perfect fluid
to be radiation, we solve the coupled equations for the system numerically and
study the evolution of the perturbations from the sub-Hubble to the
super-Hubble scales. In particular, we analyze the effects of the transition
from tachyon driven inflation to the radiation dominated epoch on the evolution
of the large scale curvature and non-adiabatic pressure perturbations. We
consider two different tachyonic potentials and study the effects of two
possible types of decay of the tachyon into radiation. We plot the spectrum of
curvature perturbations at the end of inflation as well as at the early stages
of the radiation dominated epoch. We find that reheating does not affect the
amplitude of the curvature perturbations in any of these cases. These results
corroborate similar conclusions that have been arrived at earlier based on the
study of the evolution of the perturbations in the super-Hubble limit. We
illustrate that, before the transition to the radiation dominated epoch, the
relative non-adiabatic pressure perturbation between the tachyon and radiation
decays in a fashion very similar to that of the intrinsic entropy perturbation
associated with the tachyon. Moreover, we show that, after the transition, the
relative non-adiabatic pressure perturbation dies down extremely rapidly during
the early stages of the radiation dominated epoch. These behavior ensure that
the amplitude of the curvature perturbations remain unaffected during
reheating. We also discuss the corresponding results for the popular chaotic
inflation model in the case of the canonical scalar field. [abridged]
