The sum of the virial masses of A2163-A and A2163-B as a function of the projection angle. The horizontal lines show the cluster mass estimate (full line) and errors (dashed lines) obtained in the present paper. The projected distance of 1.25 Mpc and radial velocity difference of 200 km s^-1 are derived from Maurogordato et al. (2008). The two systems were at zero separation 10 Gyr ago (left panel), 0.5 Gyr ago (middle panel), and 1.0 Gyr ago (right panel). The dotted curve corresponds to the Newtonian criterion for gravitational binding. Bound solutions are above this plot.