  We study the existence of cocompact lattices in Lie groups with bi-invariant
metric of signature $(2,n-2)$. We assume in addition that the Lie groups under
consideration are simply-connected, indecomposable and solvable. Then their
centre is one- or two-dimensional. In both cases, a parametrisation of the set
of such Lie groups is known. We give a necessary and sufficient condition for
the existence of a lattice in terms of these parameters. For groups with
one-dimensional centre this problem is related to Salem numbers.
