  We report the discovery and confirmation of a transiting circumbinary planet
(PH1b) around KIC 4862625, an eclipsing binary in the Kepler field. The planet
was discovered by volunteers searching the first six Quarters of publicly
available Kepler data as part of the Planet Hunters citizen science project.
Transits of the planet across the larger and brighter of the eclipsing stars
are detectable by visual inspection every ~137 days, with seven transits
identified in Quarters 1-11. The physical and orbital parameters of both the
host stars and planet were obtained via a photometric-dynamical model,
simultaneously fitting both the measured radial velocities and the Kepler light
curve of KIC 4862625. The 6.18 +/- 0.17 Earth radii planet orbits outside the
20-day orbit of an eclipsing binary consisting of an F dwarf (1.734 +/- 0.044
Solar radii, 1.528 +/- 0.087 Solar masses) and M dwarf (0.378+/- 0.023 Solar
radii, 0.408 +/- 0.024 Solar masses). For the planet, we find an upper mass
limit of 169 Earth masses (0.531 Jupiter masses) at the 99.7% confidence level.
With a radius and mass less than that of Jupiter, PH1b is well within the
planetary regime. Outside the planet's orbit, at ~1000 AU,a previously unknown
visual binary has been identified that is likely bound to the planetary system,
making this the first known case of a quadruple star system with a transiting
planet.
