A hidden sector of vector-tensor gravity passes compact-star tests

Hellings-Nordtvedt theory couples a vector field to spacetime curvature via two independent terms. Demanding physically sensible asymptotic behavior at large distances turns out to forbid mixing those two terms, forcing the theory into one of two pure single-coupling sectors. The overlooked sector — coupling the vector field to the Ricci scalar — allows an asymptotically flat Schwarzschild exterior with a nontrivial radial vector field, and the Solar System bounds on it are mild enough that neutron stars in this sector can show appreciable differences from general relativity in mass, radius, and moment of inertia.