  The $n+1$-dimensional Milne Universe with extra free directions is used to
construct simple FRW cosmological string models in four dimensions, describing
expansion in the presence of matter with $p=k \rho $, $k=(4-n)/3n$. We then
consider the n=2 case and make SL(2,Z) orbifold identifications. The model is
surprisingly related to the null orbifold with an extra reflection generator.
The study of the string spectrum involves the theory of harmonic functions in
the fundamental domain of SL(2,Z). In particular, from this theory one can
deduce a bound for the energy gap and the fact that there are an infinite
number of excitations with a finite degeneracy. We discuss the structure of
wave functions and give examples of physical winding states becoming light near
the singularity.
