  For non-reciprocal objects in vacuum, the Casimir interaction can become
repulsive. Here, we present a comprehensive study for idealized non-reciprocal
materials known as perfect electromagnetic conductors (PEMC). The system
consists of two spheres made of different PEMC materials, including the
plane-sphere geometry as a particular case. The sign of the Casimir force does
not only depend on the distance between the spheres and their geometric
parameters but can be controlled by adjusting the temperature. A repulsive
Casimir interaction at small distances allows for stable equilibrium
configurations of the spheres. A sum rule previously derived for the
plane-plane geometry at zero temperature is violated in general, if at least
one plane is replaced by a sphere.
