Circular Orbits under Central Forces in Special Relativity
  We discuss the existence and stability of circular orbits of a relativistic point particle moving in a central force field. The stability condition is somewhat more restrictive in Special Relativity. In the particular case of attractive central force fields proportional to $1/r^n$, we find the values the angular momentum may have in circular orbits and show that, unlike in the non-relativistic limit, the latter are not always stable in Special Relativity for $2<n<3$. 
J M Aguirregabiria, A Hern\'andez and M Rivas