Title: Rapid rotators revisited: absolute dimensions of KOI-13,
Abstract: We analyse Kepler light-curves of the exoplanet KOI-13b transiting its
moderately rapidly rotating (gravity-darkened) parent star. A physical model,
with minimal ad hoc free parameters, reproduces the time-averaged light-curve
at the ca. 10 parts per million level. We demonstrate that this Roche-model
solution allows the absolute dimensions of the system to be determined from the
star's projected equatorial rotation speed, v(e)sin(i), without any additional
assumptions; we find a planetary radius 1.33+/-0.05 R(Jup), stellar polar
radius 1.55+/-0.06 R(sun), combined mass M(*) + M(P) (\simeq M*) = 1.47 +/-
0.17 M(sun), and distance d \simeq 370+/-25 pc, where the errors are dominated
by uncertainties in relative flux contribution of the visual-binary companion
KOI-13B. The implied stellar rotation period is within ca. 5% of the
non-orbital, 25.43-hr signal found in the Kepler photometry. We show that the
model accurately reproduces independent tomographic observations, and yields an
offset between orbital and stellar-rotation angular-momentum vectors of
60.25+/-0.05 degrees.