  Keck-telescope spectrophotometry of the companion of PSR J1810+1744 shows a
flat, but asymmetric light-curve maximum and a deep, narrow minimum. The
maximum indicates strong gravity darkening near the L_1 point, along with a
heated pole and surface winds. The minimum indicates a low underlying
temperature and substantial limb darkening. The gravity darkening is a
consequence of extreme pulsar heating and the near-filling of the Roche lobe.
Light-curve modeling gives a binary inclination i=65.7+/-0.4deg. With the
Keck-measured radial-velocity amplitude K_c=462.3+/-2.2km/s, this gives an
accurate neutron star mass M_NS=2.13+/-0.04M_o, with important implications for
the dense-matter equation of state. A classic direct-heating model, ignoring
the L_1 gravitational darkening, would predict an unphysical M_NS>3M_o. A few
other ``spider" pulsar binaries have similar large heating and fill factor;
thus, they should be checked for such effects.
