  Through the combination of high-order Adaptive Optics and coronagraphy, we
report the discovery of a faint stellar companion to the A3V star zeta
Virginis. This companion is ~7 magnitudes fainter than its host star in the
H-band, and infrared imaging spanning 4.75 years over five epochs indicates
this companion has common proper motion with its host star. Using evolutionary
models, we estimate its mass to be 0.168+/-.016 solar masses, giving a mass
ratio for this system q = 0.082. Assuming the two objects are coeval, this mass
suggests a M4V-M7V spectral type for the companion, which is confirmed through
integral field spectroscopic measurements. We see clear evidence for orbital
motion from this companion and are able to constrain the semi-major axis to be
greater than 24.9 AU, the period > 124$ yrs, and eccentricity > 0.16.
Multiplicity studies of higher mass stars are relatively rare, and binary
companions such as this one at the extreme low end of the mass ratio
distribution are useful additions to surveys incomplete at such a low mass
ratio. Moreover, the frequency of binary companions can help to discriminate
between binary formation scenarios that predict an abundance of low-mass
companions forming from the early fragmentation of a massive circumstellar
disk. A system such as this may provide insight into the anomalous X-ray
emission from A stars, hypothesized to be from unseen late-type stellar
companions. Indeed, we calculate that the presence of this M-dwarf companion
easily accounts for the X-ray emission from this star detected by ROSAT.
