  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 +/- 1.0 days, and its projected obliquity with respect
to the stellar binary orbit is 1.6 +/- 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%.
