KOI-256 is a double star located in the constellation Cygnus approximately 575ly from Earth. While observations by the Kepler spacecraft suggested the system contained a gas giant exoplanet orbiting a red dwarf, later studies determined that KOI-256 was a binary system composed of the red dwarf orbiting a white dwarf.
The acronym "KOI" comes from Kepler object of interest and means that the object has been cataloged by the Kepler spacecraft during its search for extrasolar planets using the transit method. The "256" is the number of the object.
Initial observations by the Kepler spacecraft suggested a central red dwarf with a mass of, a radius of, and a temperature of 3639K. Its candidate exoplanet was estimated to have a mass of, a radius of, an orbital period of 1.38 days, a temperature of 1160K, and a semi-major axis of 0.021 astronomical units. Further studies by Muirhead et al. (2012) refined the candidate exoplanet parameters to a radius of, a temperature of 726K, and a semi-major axis of 0.016 AU.
Muirhead et al. (2013) performed additional observations with the Hale Telescope at Palomar Observatory. Using the radial velocity method for exoplanet detection, Muirhead's team found that the red dwarf was wobbling too much to be caused by a planetary mass object, and was more likely being influenced by a white dwarf. Using ultraviolet data from the GALEX spacecraft, it was seen that the red dwarf was significantly active, further suggesting perturbations by a white dwarf. The team re-analyzed Kepler data, and found that when the white dwarf passed in front of the red dwarf, the red dwarf's light noticeably warped and brightened, an effect called gravitational lensing. While only being slightly larger than the Earth, the white dwarf has such large mass that the physically larger red dwarf orbits around its smaller companion.
With the new observations, the red dwarf was shown to have a mass of, a radius of, and a temperature of 3450±. The white dwarf has a mass of, a radius of, and a temperature of 7100±.