2017MNRAS.470..932H

We analyse Kepler light-curves of the exoplanet Kepler Object of Interest no. 13b (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 ∼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 R_P = (1.33 ± 0.05) R, stellar polar radius R_p = (1.55 ± 0.06) R_☉, combined mass M_* + M_P( ≃ M_*) = (1.47 ± 0.17) M_☉ and distance d ≃ (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 ∼5 per cent 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.