  Recent work on increasing the effective number of neutrino species (Neff) in
the early universe has focussed on introducing extra relativistic species
(`dark radiation'). We draw attention to another possibility: a new particle of
mass less than 10 MeV that remains in thermal equilibrium with neutrinos until
it becomes non-relativistic increases the neutrino temperature relative to the
photons. We demonstrate that this leads to a value of Neff that is greater than
three and that Neff at CMB formation is larger than at BBN. We investigate the
constraints on such particles from the primordial abundance of helium and
deuterium created during BBN and from the CMB power spectrum measured by ACT
and SPT and find that they are presently relatively unconstrained. We forecast
the sensitivity of the Planck satellite to this scenario: in addition to
dramatically improving constraints on the particle mass, in some regions of
parameter space it can discriminate between the new particle being a real or
complex scalar.
