Proper motion diagram of USco photometric member candidates. The seven photometric candidates are labelled and depicted in red, while the two candidates rejected in Sect. 3.3 (also labelled) are shown in gray. The gray ellipse centered at (−12.1, −23.8) mas yr^-1 denotes our 2σ astrometric criterion to identify objects with proper motion, consistent with USco membership, where σ is the astrometric dispersion of USco low-mass members (see text). The sources used as astrometric references are presented by black dots. The high proper motion source 2MASS J15501151−2201213 (first discovered by Deacon et al. 2009) lies off limits.