  We investigate the effects of the light vector U-boson that couples weakly to
nucleons in relativistic mean-field models on the equation of state and
subsequently the consequence in neutron stars. It is analyzed that the U-boson
can lead to a much clearer rise of the neutron star maximum mass in models with
the much softer equation of state. The inclusion of the U-boson may thus allow
the existence of the non-nucleonic degrees of freedom in the interior of large
mass neutron stars initiated with the favorably soft EOS of normal nuclear
matter. In addition, the sensitive role of the U-boson in the neutron star
radius and its relation to the test of the non-Newtonian gravity that is herein
addressed by the light U-boson are discussed.
