Holonomy of pseudo-Riemannanian manifolds with signature (n,n)

We study the holonomy group of a pseudo-riemannian manifold with signature (n, n). We first study metrics whose holonomy group leaves invariant two n-dimensional totally isotropic complementary subspaces. Then we give a list of candidates for restricted holonomy groups of indecomposable non-irreducible pseudo-riemannian manifolds with signature (2, 2), and some examples of related metrics.