WISE J104915.57 - 531906.1 A+B and WISE J085510.83 - 071442.5 were recently discovered as the third and fourth closest known systems to the Sun , respectively ( 2.0 and 2.3 pc ) .
The former consists of a L8+T0.5 binary and the latter is a probable Y dwarf and is the coldest known brown dwarf ( \sim 250 K ) .
We present a search for common proper motion companions to these brown dwarfs using multi-epoch mid-infrared images from the Spitzer Space Telescope .
We have also obtained near-infrared adaptive optics images of WISE J104915.57 - 531906.1 A+B with the Very Large Telescope to search for companions at smaller separations than reached by Spitzer .
No new companions are detected in either system .
At projected separations of 25–420 \arcsec ( 50–840 AU ) for WISE J104915.57 - 531906.1 A+B and 4–420 \arcsec ( 9–970 AU ) for WISE J085510.83 - 071442.5 , the Spitzer images are sensitive to companions with M _ { 4.5 } \lesssim 21.6 and 21.9 , respectively , which correspond to masses of \gtrsim 1 M _ { Jup } for ages of \geq 1 Gyr and temperatures of \gtrsim 150 K. The detection limit in the adaptive optics images of WISE J104915.57 - 531906.1 A+B is \Delta H \sim 10 at 3– 15 \arcsec ( 6–30 AU ) , or \gtrsim 7 M _ { Jup } for \geq 1 Gyr .