RXJ0942.7-7726AB: An isolated pre-main-sequence wide binary

We report the discovery of two young M-dwarfs, RXJ0942.7-7726 (M1) and 2MASSJ09424157-7727130 (M4.5), that were found only 42arcsec apart in a survey for pre-main-sequence stars surrounding the open cluster ηChamaeleontis. Both stars have congruent proper motions and near-infrared photometry. Medium-resolution spectroscopy reveals that they are coeval (age 8-12Myr), codistant (100-150pc) and thus almost certainly form a true wide binary with a projected separation of 4000-6000au. The system appears too old and dynamically fragile to have originated in ηCha and a trace-back analysis argues for its birth in or near the Scorpius-Centaurus OB association (Sco-Cen). RXJ0942.7-7726AB joins a growing group of wide binaries kinematically linked to Sco-Cen, suggesting that such fragile systems can survive the turbulent environment of their natal molecular clouds while still being dispersed with large velocities. Conversely, the small radial velocity difference between the stars (2.7 ± 1.0kms^-1) could mean the system is unbound, a result of the coincidental ejection of two single stars with similar velocity vectors from the OB association early in its evolution.