We report the detection of a faint stellar companion to the famous nearby A5V star Alcor (80 UMa). The companion has M-band (λ = 4.8 μm) magnitude 8.8 and projected separation 1. 11 (28 AU) from Alcor. The companion is most likely a low-mass (∼0.3 M ) active star which is responsible for Alcor's X-ray emission detected by ROSAT (L X 10 28.3 erg s −1 ). Alcor is a nuclear member of the Ursa Major star cluster (UMa; d 25 pc, age 0.5 Gyr), and has been occasionally mentioned as a possible distant

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... 709 ) companion of the stellar quadruple Mizar (ζ 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 −1 , which is comparable to the predicted velocity amplitude induced by the newly discovered companion (∼1 km s −1 ). 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 ± 0.19 pc (74 ± 39 kAU; minimum 18 kAU), and their velocity vectors are marginally consistent (χ 2 probability 6%). Given their close proximity and concordant motions we suggest that the Mizar quadruple and the Alcor binary be together considered the second 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). from adopted T eff and tables of Flower 1996; (7) from adopted V magnitude and parallax, assuming zero extinction; (8) from adopted M V and BC V ; (9) soft X-ray luminosity (0.2-2.4 keV) in ROSAT band, calculated using count rate and hardness ratio HR1 from Voges et al. 2000, energy conversion factor from Fleming et al. 1995, and the adopted parallax; (10) combining T eff and log(L/L ) values with z = 0.02 evolutionary tracks of Lejeune & Schaerer 2001; (11) UMa cluster age from King et al. 2003; (12) Galactic Cartesian velocity vector, calculated in Section 3.4.1.