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 \sim 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 ^ { + .012 } _ { - .016 } M _ { \odot } , giving a mass ratio for this system q = 0.082 ^ { + .007 } _ { - .008 } .
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 \gtrsim 24.9 AU , the period \gtrsim 124 yrs , and eccentricity \gtrsim 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 .