Constraints on millicharged particles from Planck data

We revisit cosmic microwave background constraints on the abundance of millicharged particles based on the Planck data. The stringent limit [Omega] sub(mcp)h super(2) < 0.001 (95% CL) may be set using the cosmic microwave background data alone if millicharged particles participate in the acoustic oscillations of baryon-photon plasma at the recombination epoch. The latter condition is valid for a wide region of charges and masses of the particles. Adding the millicharged component to [Lambda] CDM shifts the preferred scalar spectral index of primordial perturbations to somewhat larger values as compared to the minimal model, even approaching the Harrison-Zeldovich spectrum under some assumptions.