The null Killing surfaces for the Kerr-Newman solution to the Einstein field equations is discussed, derived and plotted. For some parameters of the mass m, the angular momentum per unit mass a, and the charge e, the plots show some very unusual features.
https://www.gemarsh.com/wp-content/uploads/Null-Killing-Surf-of-the-Kerr-Kerr-Newman-Sol-1.pdf
The Null-Killing Surfaces of the Kerr-Newman Solutions of the Einstein Field Equations