Left panels: Keck-I/OSIRIS Adaptive Optics images of the primaries and secondaries of USco1621 and USco1556 systems, in the K_p filter. The field-of-view of the images is 1′′× 1′′ and the orientation is north up and east to the left. Contrast scale of the images has been adjusted individually. Right panels: contrast curves for the primaries and secondaries of the USco1621 and USco1556 systems for a 3σ detection limit. The expected differential magnitudes for objects at the theoretical mass limits for hydrogen and deuterium burning are marked by dashed lines.