A new tensorial conservation law for Maxwell fields on the Kerr   background
  A new, conserved, symmetric tensor field for a source-free Maxwell test field on a four-dimensional spacetime with a conformal Killing-Yano tensor, satisfying a certain compatibility condition, is introduced. In particular, this construction works for the Kerr spacetime. 
Lars Andersson, Thomas B\"ackdahl and Pieter Blue
gr-qc math.AP