The effect of cosmic rays on the instability of a tangential discontinuity

The Kelvin-Helmholtz instability of a tangential velocity discontinuity in a plasma flow when cosmic-ray pressure is taken into account is studied. It is shown that a new unstable mode can exist. This appears when an effective Mach number exceeds some critical value. In the case of strong flow mediation by cosmic rays the wave vectors of the most-unstable waves are directed along the plasma velocity vector for both subsonic and supersonic flows.