  We build a realistic model of curvaton cosmology, in which the energy content
is described by radiation, WIMP dark matter and a curvaton component. We
calculate the curvature and isocurvature perturbations, allowing for arbitrary
initial density perturbations in all fluids, following all species and their
perturbations from the onset of dark matter freeze-out onto well after curvaton
decay. We provide detailed numerical evaluations as well as analytical formulae
which agree well with the latter. We find that substantial isocurvature
perturbations, as measured relatively to the total curvature perturbation, can
be produced even if the curvaton energy density is well underdominant when it
decays; high precision measurements of cosmic microwave background anisotropies
may thus open a window on underdominant decoupled species in the
pre-nucleosynthesis early Universe. We also find that in a large part of
parameter space, curvaton decay produces enough dark matter particles to
restore WIMP annihilations, leading to the partial erasure of any pre-existing
dark matter - radiation isocurvature perturbation.
