  Gravitational S-duality is defined by the contraction of two indices of the
Riemann tensor with the epsilon tensor. We review its realization in linearized
gravity, and study its generalization to full non-linear gravity by means of
explicit examples: Up to a rescaling of the coordinates, it relates two
Taub-NUT-Schwarzschild metrics by interchanging m with l, provided both
parameters are non-zero. In the presence of a cosmological constant
gravitational S-duality can be implemented at the expense of the introduction
of a three-form field whose value turns out to be dual to the cosmological
constant.
