  With appropriate hypotheses on the nonlinearity $f$, we prove the existence
of a ground state solution $u$ for the problem \[(-\Delta+m^2)^\sigma
u+Vu=\left(W*F(u)\right)f(u)\ \ \text{in }\ \mathbb{R}^{N},\] where
$0<\sigma<1$, $V$ is a bounded continuous potential and $F$ the primitive of
$f$. We also show results about the regularity of any solution of this problem.
