Reaching the boundary between stellar kinematic groups and very wide binaries II. α Librae plus KU Librae: a common proper motion system in Castor separated by 1.0 pc

Aims. I investigate the gravitational binding of a nearby common proper motion system in the young Castor moving group (tau similar to 200 Ma), which is formed by the bright quadruple star alpha Lib (Zubenelgenubi) and the young solar analogue KU Lib. The system has an exceptionally wide angular separation of about 2.6 deg, which corresponds to a projected physical separation of about 1.0 pc. Methods. I compiled basic information on the system, compared its binding energy with those of other weakly bound systems in the field, and studied the physical separations of resolved multiple systems in Castor. Results. KU Lib has roughly the same proper motion, parallactic distance, radial velocity, and metallicity as the young hierarchical quadruple system alpha Lib. It also displays youth features. The resemblance between these basic parameters and the relatively high estimated binding energy indicate that the five stars are gravitationally bound. KU Lib and alpha Lib constitute the widest known multiple system in all mass domains, and probably represent the most extreme example of young wide binaries on the point of being disrupted. Besides this, I make a comprehensive compilation of star candidates in Castor, including new ones.