We report the detection of a faint stellar companion to the famous nearby A5V star Alcor ( 80 UMa ) .
The companion has M-band ( \lambda = 4.8 \mu m ) magnitude 8.8 and projected separation 1 ” .11 ( 28 AU ) from Alcor .
The companion is most likely a low-mass ( \sim 0.3 M _ { \odot } ) active star which is responsible for Alcor ’ s X-ray emission detected by ROSAT ( L _ { X } \simeq 10 ^ { 28.3 } erg/s ) .
Alcor is a nuclear member of the Ursa Major star cluster ( UMa ; d \simeq 25 pc , age \simeq 0.5 Gyr ) , and has been occasionally mentioned as a possible distant ( 709 ” ) companion of the stellar quadruple Mizar ( \zeta UMa ) .
Comparing the revised Hipparcos proper motion for Alcor with the mean motion for other UMa nuclear members shows that Alcor has a peculiar velocity of 1.1 km/s , which is comparable to the predicted velocity amplitude induced by the newly-discovered companion ( \sim 1 km/s ) .
Using a precise dynamical parallax for Mizar and the revised Hipparcos parallax for Alcor , we find that Mizar and Alcor are physically separated by 0.36 \pm 0.19 pc ( 74 \pm 39 kAU ; minimum 18 kAU ) , and their velocity vectors are marginally consistent ( \chi ^ { 2 } probability 6 % ) .
Given their close proximity and concordant motions we suggest that the Mizar quadruple and the Alcor binary be together considered the 2nd closest stellar sextuplet .
The addition of Mizar-Alcor to the census of stellar multiples with six or more components effectively doubles the local density of such systems within the local volume ( d < 40 pc ) .