  We characterise the primordial perturbations produced due to both inflaton
and curvaton fluctuations in models where the curvaton has a quadratic, cosine
or hyperbolic potential, and the inflaton potential is characterised by the
usual slow-roll parameters. Isocurvature curvaton field perturbations can
produce significant non-Gaussianity in the primordial density field, in
contrast with adiabatic inflaton field perturbations which produce negligible
non-Gaussianity for canonical scalar fields. A non-self-interacting curvaton
with quadratic potential produces a local-type non-Gaussianity that is well
described by the non-linearity parameter fNL, which may be scale-dependent when
the inflaton perturbations dominate the power spectrum. We show how
observational bounds on non-linearity parameters and the tensor-scalar ratio
can be used to constrain curvaton and inflaton parameters. We find a
consistency relation between the bispectrum and trispectrum parameters in a
mixed inflaton-curvaton model for a quadratic curvaton potential.
Self-interaction terms in the curvaton potential can lead to both a large
trispectrum parameter, gNL, and scale-dependence of the non-linearity
parameters.
