  Highly-excited states of excitons in cuprous oxide have recently been
observed at a record quantum number of up to $n=25$. Here, we evaluate the
long-range interactions between pairs of Rydberg excitons in Cu$_2$O, which are
due to direct Coulomb forces rather than short-range collisions typically
considered for ground state excitons. A full numerical analysis is supplemented
by the van der Waals asymptotics at large exciton separations, including the
angular dependence of the potential surfaces.
