Solutions of Penrose's Equation
  The computational use of Killing potentials which satisfy Penrose's equation is discussed. Penrose's equation is presented as a conformal Killing-Yano equation and the class of possible solutions is analyzed. It is shown that solutions exist in spacetimes of Petrov type O, D or N. In the particular case of the Kerr background, it is shown that there can be no Killing potential for the axial Killing vector. 
E.N. Glass (Department of Physics, University of Michigan, Ann Arbor,   MI) and Jonathan Kress (School of Mathematics and St...