Solar system constraints on R$^n$ gravity
  Recently, gravitational microlensing has been investigated in the framework of the weak field limit of fourth order gravity theory. However, solar system data (i.e. planetary periods and light bending) can be used to put strong constraints on the parameters of this class of gravity theories. We find that these parameters must be very close to those corresponding to the Newtonian limit of the theory. 
A.F. Zakharov, A.A. Nucita, F. De Paolis and G. Ingrosso