  We report the discovery of a new Kepler transiting circumbinary planet (CBP).
This latest addition to the still-small family of CBPs defies the current trend
of known short-period planets orbiting near the stability limit of binary
stars. Unlike the previous discoveries, the planet revolving around the
eclipsing binary system Kepler-1647 has a very long orbital period (~1100 days)
and was at conjunction only twice during the Kepler mission lifetime. Due to
the singular configuration of the system, Kepler-1647b is not only the
longest-period transiting CBP at the time of writing, but also one of the
longest-period transiting planets. With a radius of 1.06+/-0.01 RJup it is also
the largest CBP to date. The planet produced three transits in the light-curve
of Kepler-1647 (one of them during an eclipse, creating a syzygy) and
measurably perturbed the times of the stellar eclipses, allowing us to measure
its mass to be 1.52+/-0.65 MJup. The planet revolves around an 11-day period
eclipsing binary consisting of two Solar-mass stars on a slightly inclined,
mildly eccentric (e_bin = 0.16), spin-synchronized orbit. Despite having an
orbital period three times longer than Earth's, Kepler-1647b is in the
conservative habitable zone of the binary star throughout its orbit.
