  We report the detection of the first circumbinary planet found by TESS. The
target, a known eclipsing binary, was observed in sectors 1 through 12 at
30-minute cadence and in sectors 4 through 12 at two-minute cadence. It
consists of two stars with masses of 1.1 MSun and 0.3 MSun on a slightly
eccentric (0.16), 14.6-day orbit, producing prominent primary eclipses and
shallow secondary eclipses. The planet has a radius of ~6.9 REarth and was
observed to make three transits across the primary star of roughly equal depths
(~0.2%) but different durations -- a common signature of transiting
circumbinary planets. Its orbit is nearly circular (e ~ 0.09) with an orbital
period of 95.2 days. The orbital planes of the binary and the planet are
aligned to within ~1 degree. To obtain a complete solution for the system, we
combined the TESS photometry with existing ground-based radial-velocity
observations in a numerical photometric-dynamical model. The system
demonstrates the discovery potential of TESS for circumbinary planets, and
provides further understanding of the formation and evolution of planets
orbiting close binary stars.
