Non-perturbative and self-consistent models of neutron stars in   R-squared gravity
  In the present paper we investigate non-perturbatively and self-consistently the structure of neutron stars in $R$-squared gravity by simultaneously solving the interior and exterior problem. The mass-radius relations are obtained for several equations of state and for wide range of the $R$-squared gravity parameter $a$. Even though the deviation from general relativity for nonzero values of $a$ can be large, they are still comparable with the variations due to different modern realistic equations of state. That is why the current observations of the neutron star masses and radii alone can not put constraints on the value of the parameter $a$. We also compare our results with those obtained within the perturbative method and we discuss the differences between them. 
Stoytcho S. Yazadjiev, Daniela D. Doneva, Kostas D. Kokkotas and Kalin   V. Staykov
gr-qc astro-ph.HE