  Recent cosmological observations, including measurements of the CMB
anisotropy and the primordial helium abundance, indicate the existence of an
extra radiation component in the Universe beyond the standard three neutrino
species. In this paper we explore the possibility that the extra radiation has
isocurvatrue fluctuations. A general formalism to evaluate isocurvature
perturbations in the extra radiation is provided in the mixed inflaton-curvaton
system, where the extra radiation is produced by the decay of both scalar
fields. We also derive constraints on the abundance of the extra radiation and
the amount of its isocurvature perturbation. Current observational data favors
the existence of an extra radiation component, but does not indicate its having
isocurvature perturbation. These constraints are applied to some particle
physics motivated models. If future observations detect isocurvature
perturbations in the extra radiation, it will give us a hint to the origin of
the extra radiation.

Cosmology and Nongalactic Astrophysics;High Energy Physics - Phenomenology