Spherical photon orbits around a Kerr black hole
For a rotating Kerr black hole there are two circular photon orbits that could exist in the equatorial plane. A Schwarzschild black hole has also such orbits. But in the Kerr black hole the spherical timelike orbits were found by Wilkins (1972). In the paper the author focuses on spherical photon orbits with positive energy outside the event horizon with the aim of finding and studying all such orbits.
scientific article; zbMATH DE number 2031222