Discrete symmetries of Petrov type-I space-times

The arbitrariness in the definition of the canonical frame in a Petrov type-I space-time is examined and shown to imply that the Newman-Penrose formalism is invariant under a 24-element discrete symmetry group. The transformations of the N-P scalars under the operations of the group are obtained, and conditions are formulated which characterize space-times that maintain this symmetry globally.