Spacetime characterizations of $Λ$-vacuum metrics with a null Killing 2-form
An exhaustive list of four-dimensional $Λ$-vacuum spacetimes admitting a Killing vector whose self-dual Killing two-form ${\cal F}$ is null is obtained assuming that the self-dual Weyl tensor is proportional to the tensor product of ${\cal F}$ by itself. Our analysis complements previous results concerning the case with non-null ${\cal F}$. We analyze both cases with $Λ$ vanishing or not. In the latter case we prove that $Λ< 0$ must hold necessarily, and we find a characterization of the Einstein spacetimes conformal to pp-waves. In the former case we obtain spacetime characterizations of vacuum plane waves and of the stationary vacuum Brinkmann spacetimes. At the light of the full set of results, old and new, we reformulate the case with non-null ${\cal F}$ and $Λ= 0$. We finally present a table collecting the results for both null, and non-null, ${\cal F}$.