Maximally supersymmetric solutions of R² supergravity

There are five maximally supersymmetric backgrounds in four-dimensional off-shell N=1 supergravity, two of which are well known: Minkowski superspace M^{4|4} and anti-de Sitter superspace AdS^{4|4}. The three remaining supermanifolds support spacetimes of different topology, which are: R x S^3, AdS_3 x R, and a supersymmetric plane wave isometric to the Nappi-Witten group. As is well known, the Minkowski and anti-de Sitter superspaces are solutions of the Poincar\'e and anti-de Sitter supergravity theories, respectively. Here we demonstrate that the other three superspaces are solutions of no-scale $R^2$ supergravity. We also present a new (probably the simplest) derivation of the maximally supersymmetric backgrounds of off-shell N=1 supergravity.