Massive sterile neutrinos as warm dark matter

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 sin22θ>10−11 for mixing with νμ or ντ, while mixing with νe is slightly stronger bounded with mass less than 30 keV.