Relativistic stars in vector-tensor theories
  We study relativistic star solutions in second-order generalized Proca
theories characterized by a $U(1)$-breaking vector field with derivative
couplings. In the models with cubic and quartic derivative coupling, the mass
and radius of stars become larger than those in general relativity for negative
derivative coupling constants. This phenomenon is mostly attributed to the
increase of star radius induced by a slower decrease of the matter pressure
compared to general relativity. There is a tendency that the relativistic star
with a smaller mass is not gravitationally bound for a low central density and
hence dynamically unstable, but that with a larger mass is gravitationally
bound. On the other hand, we show that the intrinsic vector-mode couplings give
rise to general relativistic solutions with a trivial field profile, so the
mass and radius are not modified from those in general relativity.
