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We investigate the linear regime of f(R) = R + alpha R^2 gravity for static,spherically symmetric and asymptotically flat configurations of matter. We showthat, in vacuum and deep inside the range of the extra scalar degree offreedom, the post-Newtonian parameter gamma is not equal to 1/2, as establishedin the literature, but it assumes larger values depending on the pressure ofthe star. We provide an explicit expression for gamma in terms of the mass, ofthe integrated pressure of the star and of the ratio between the star's radiusand the range of the extra degree of freedom. We corroborate our results byproviding numerical solutions for the case of a neutron star.

Pressure effects in the weak-field limit of f(R) = R + alpha R^2 gravity