  The A5V star Alcor has an M3-M4 dwarf companion, as evidenced by a novel
astrometric technique. Imaging spectroscopy combined with adaptive optics
coronagraphy allowed for the detection and spectrophotometric characterization
of the point source at a contrast of ~6 J- and H-band magnitudes and separation
of 1" from the primary star. The use of an astrometric pupil plane grid allowed
us to determine the projected separations between the companion and the
coronagraphically occulted primary star to <=3 milliarcsecond precision at two
observation epochs. Our measurements demonstrate common parallactic and proper
motion over the course of 103 days, significantly shorter than the period of
time needed for most companion confirmations through proper motion measurements
alone. This common parallax method is potentially more rigorous than common
proper motion, ensuring that the neighboring bodies lie at the same distance,
rather than relying on the statistical improbability that two objects in close
proximity to each other on the sky move in the same direction. The discovery of
a low-mass (~0.25M_sun) companion around a bright (V = 4.0), nearby (d = 25 pc)
star highlights a region of binary star parameter space that to date has not
been fully probed.
