Classification of closed conformally flat Lorentzian manifolds with unipotent holonomy

We classify closed, conformally flat Lorentzian manifolds of dimension $n\ge 3$ with unipotent holonomy in PO(2,n). They are all Kleinian and fall into four different geometric types according to the intersection of the image of the developing map with a holonomy-invariant isotropic flag. They are homeomorphic to $S^{n-1}\times S^1$ or a nilmanifold of degree at most three, up to a finite cover. We classify those admitting an essential conformal flow; these fall into two geometric types, both homeomorphic to $S^{n-1}\times S^1$ up to finite cover.