  The Cotton-York and Simon-Mars tensors in stationary vacuum spacetimes are
studied in the language of the congruence approach pioneered by Hawking and
Ellis. Their relationships with the Papapetrou field defined by the stationary
Killing congruence and with a recent characterization of the Kerr spacetime in
terms of the alignment between of the principal null directions of the Weyl
tensor with those of the Papapetrou field are also investigated in this more
transparent language.
