  We discuss the impact of a hidden sector consisting of Minicharged Particles
(MCPs) and massless hidden photons on the expansion history of our Universe. We
present parameter scans for the amount of extra relativistic particles (Neff)
and the abundance of light nuclei for fermionic MCPs with masses between ~100
keV and 10 GeV and minicharges in the range 10^(-11)-1. Current CMB and BBN
data significantly constrain the available parameter space of MCPs. The shown
results are a valuable indicator for future experimental searches and are
presented in a flexible way so that more accurate results on Neff can be easily
interpreted.
