We show that massive sterile neutrinos mixed with the ordinary ones may be produced in the early universe in the right amount to be natural warm dark matter particles. Their mass should be below 40 keV and the corresponding mixing angles sin 2 2θ > 10 −11 for mixing with ν µ or ν τ , while mixing with ν e is slightly stronger bounded with mass less than 30 keV.

We revisit cosmic microwave background (CMB) constraints on the abundance of millicharged particles based on the Planck data. The stringent limit Ωmcph 2 < 0.001 (95% CL) may be set using the CMB 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 ΛCDM shifts prefered scalar spectral index of primordial perturbations to somewhat larger values as compared to minimal model, even approaching Harrison-Zeldovich spectrum under some assumptions.