Constant angle null hypersurfaces

In this work, we introduce the notion of constant angle null hypersurface of a Lorentzian manifold with respect to a given ambient vector field. We analyze the case in which the vector field is closed and conformal, thus finding that such null hypersurfaces induce a canonical principal direction. We further provide some characterizations in the case of a parallel vector field or a four dimensional ambient space of constant sectional curvature.