  We investigate the effects of a mixing of active and sterile neutrinos on the
ratios of supernova electron neutrino flux ($F_e$) and antineutrino flux
($F_{\bar e}$) to the total flux of the other neutrino and antineutrino
flavours ($F_a$). We assume that the heaviest (in the normal hierarchy)
Standard Model neutrino $\nu_3$ mixes with a sterile neutrino resulting in a
pair of mass eigenstates with a small mass gap. Using the density matrix
formalism we solve numerically the the evolution of neutrino states in the
envelope of a supernova and determine the flux ratios $F_e/F_a$ and
$F_{\bar{e}}/F_a$ as a function of the active-sterile mixing angle and for the
experimentally allowed range of the standard active-active mixing angle
$\theta_{13}$.
