Model companions of theories with an automorphism
  For a theory $T$ in $L, T_\sigma$ is the theory of the models of $T$ with an automorphism $\sigma$. If $T$ is an unstable model complete theory without the independence property, then $T_\sigma$ has no model companion. If $T$ is an unstable model complete theory and $T_\sigma$ has the amalgamation property, then $T_\sigma$ has no model companion. If $T$ is model complete and has the fcp, then $T_\sigma$ has no model completion. 
Hirotaka Kikyo