Huygens' Principle for the Non-Self-Adjoint Scalar Wave Equation on   Petrov type III Space-Times
  We prove that if the non-self-adjoint scalar wave equation satisfies Huygens' principle on Petrov type III space-times, then it is equivalent to the conformally invariant scalar wave equation. 
W. G. Anderson, R. G. McLenaghan and F. D. Sasse