Newton's constant in f(R,R_{\mu \nu}R^{\mu \nu},\Box R) theories of
  gravity and constraints from BBN
  We consider corrections to the Einstein-Hilbert action which contain both
higher order and nonlocal terms. We derive an effective Newtonian gravitational
constant applicable at the weak field limit and use the primordial
nucleosynthesis (BBN) bound and the local gravity constraints on $G_{eff}$ in
order to test the viability of several cases of our general Lagrangian. We will
also provide a BBN constrain on the $\Box R$ gravitational correction.
