  Worldsheet string theory is solvable for a variety of backgrounds involving
Neveu-Schwarz fivebranes, in terms of gauged nonlinear sigma models on group
manifolds. We compute the worldsheet torus partition function of these models,
and propose gauging of null isometries as a unifying principle and conceptual
framework for this large family of string backgrounds. In the process, we
explain how partition functions of asymmetrically gauged Wess-Zumino-Witten
models can be computed from the path integral, and organize and systematize
various results scattered throughout the literature.
