We report the discovery of WISE 2150-7520AB (W2150AB): a widely separated (~341au) very low-mass L1+T8 co-moving system. The system consists of the previously known L1 primary 2MASS J21501592-7520367 and a newly discovered T8 secondary found at position 21:50:18.99-75:20:54.6 (MJD=57947) using Wide-field Infrared Survey Explorer data via the Backyard Worlds: Planet 9 citizen science project. We present Spitzer ch1 and ch2 photometry (ch1-ch2=1.41+/-0.04mag) of the secondary and Folded-port InfraRed Echellette prism spectra of both components. The sources show no peculiar spectral or photometric signatures, indicating that each component is likely field age. Using all observed data and the Gaia DR2 parallax of 41.3593+/-0.2799mas for W2150A we deduce fundamental parameters of log(L_bol_/L_{sun})=-3.69+/-0.01, Teff=2118+/-62K, and an estimated mass=72+/-12M_Jup for the L1 and log(L_bol_/L_{sun})=-5.64+/-0.02, Teff=719+/-61K, and an estimated mass=34+/-22M_Jup for the T8. At a physical separation of ~341au this system has E_bin_=10^41^erg, making it the lowest binding energy system of any pair with M_tot_<0.1M_{sun} not associated with a young cluster. It is equivalent in estimated mass ratio, E_bin, and physical separation to the ~2Myr M7.25+M8.25 binary brown dwarf 2MASS J11011926-7732383AB (2M1101AB) found in the Chameleon star-forming region. W2150AB is the widest companion system yet observed in the field where the primary is an L dwarf or later.

Cone search capability for table J/ApJ/889/176/table2 (Companion systems with total mass <0.2M_Sun_ (System properties))

Cone search capability for table J/ApJ/889/176/table3 (Companion systems with total mass <0.2M_Sun_ (Dynamics))