Gravito-electromagnetic Effects of Massive Rings
  The Einstein field equations in linear post-Newtonian approximation can be
written in analogy with electromagnetism, in the so-called
gravito-electromagnetic formalism. We use this analogy to study the
gravitational field of a massive ring: in particular, we consider a continuous
mass distribution on Keplerian orbit around a central body, and we work out the
gravitational field generated by this mass distribution in the intermediate
zone between the central body and the ring, focusing on the gravito-magnetic
component that originates from the rotation of the ring. In doing so, we
generalize and complement some previous results that focused on the purely
Newtonian effects of the ring (thus neglecting its rotation) or that were
applied to the case of rotating spherical shells. Eventually, we study in some
simple cases the effect of the the rotation of the ring, and suggest that, in
principle, this approach could be used to infer information about the angular
momentum of the ring.
