  A topological phase transition in two-dimensional nonlinear sigma-models on
tori, connected with self-dual (unimodular) 24-dimensional Niemeier lattices,
is considered. It is shown that critical properties of these transitions are
determined by corresponding Coxeter numbers of lattices. A case of general
integer-valued lattices with minimal norm equal 1 or 2 and a possible
application to string theory are discussed.
