  Kepler-13Ab (KOI-13) is an exoplanet orbiting a rapidly rotating A-type star.
The system shows a significant spin-orbit misalignment and a changing transit
duration most probably caused by the precession of the orbit. Here we present a
self-consistent analysis of the system combining {\it Kepler} and TESS
observations. We model the light curves asssuming a planet transits a rotating
oblate star which has a strong surface temperature gradient due to
rotation-induced gravity darkening. The transit chord moves slowly as an
emergent feature of orbital precession excited by the oblate star with a
decline rate in the impact parameter of ${\rm d}b/{\rm d}t = -0.011/{\rm yr}$,
and with an actual value of $b=0.19$ for the latest TESS measurements. The
changing transit duration that was measured from {\it Kepler} Q2 and Q17
quarters and the TESS measurements indicates a linear drift of the impact
parameter. The solutions for the stellar spin axis suggest a nearly orthogonal
aspect, with inclination around $100^\circ$.
