Vacuum energy in the theory of a noncritical compactified bosonic string

The single-loop vacuum energy of a free noncritical bosonic string compactified onto a torus is calculated. The modular-invariant nature of the result for any dimension $D$ is discussed. In the case of a onedimensional torus, the Hagedorn temperature is obtained. Inclusion of a Wilson loop in the theory of an open noncritical compactified string is also discussed.