  If the inflaton decays into several components during reheating, and if the
corresponding decay rates are functions of spacetime-dependent quantities, it
is possible to generate entropy perturbations after a stage of single-field
inflation. In this paper, I present a simple toy example that illustrates this
possibility. In the example, the decay rates of the inflaton into ``matter''
and ``radiation'' are different functions of the total energy density. In
particular cases, one can exactly solve the equations of motion both for
background and perturbations in the long-wavelength limit, and show that
entropy perturbations do indeed arise. Beyond these specific examples, I
attempt to identify what are the essential ingredients responsible for the
generation of entropy perturbations after single-field inflation, and to what
extent these elements are expected to be present in realistic models.

Astrophysics