Debye Potentials for Maxwell and Dirac Fields from a Generalisation of   the Killing-Yano Equation
  By using conformal Killing-Yano tensors, and their generalisations, we obtain scalar potentials for both the source-free Maxwell and massless Dirac equations. For each of these equations we construct, from conformal Killing-Yano tensors, symmetry operators that map any solution to another. 
I. M. Benn, Philip Charlton and Jonathan Kress