Light sterile neutrino in the minimal extended seesaw

Motivated by the recent observations on sterile neutrinos, we present a minimal extension of the canonical type-I seesaw by adding one extra singlet fermion. After the decoupling of right-handed neutrinos, an eV-scale mass eigenstate is obtained without the need of artificially inserting tiny mass scales or Yukawa couplings for sterile neutrinos. In particular, the active-sterile mixing is predicted to be of the order of 0.1. Moreover, we show a concrete flavor A4 model, in which the required structures of the minimal extended seesaw are realized. We also comment on the feasibility of accommodating a keV sterile neutrino as an attractive candidate for warm dark matter.