Kepler-16 is an eccentric low-mass eclipsing binary with a circumbinary transiting planet . Here we investigate the angular momentum of the primary star , based on Kepler photometry and Keck spectroscopy . The primary star ’ s rotation period is 35.1 \pm 1.0 days , and its projected obliquity with respect to the stellar binary orbit is 1.6 \pm 2.4 degrees . Therefore the three largest sources of angular momentum—the stellar orbit , the planetary orbit , and the primary ’ s rotation—are all closely aligned . This finding supports a formation scenario involving accretion from a single disk . Alternatively , tides may have realigned the stars despite their relatively wide separation ( 0.2 AU ) , a hypothesis that is supported by the agreement between the measured rotation period and the “ pseudosynchronous ” period of tidal evolution theory . The rotation period , chromospheric activity level , and fractional light variations suggest a main-sequence age of 2-4 Gyr . Evolutionary models of low-mass stars can match the observed masses and radii of the primary and secondary stars to within about 3 % .