Most string theory compactifications analyzed therefore far have as backgrounds a conformal field theory corresponding to a nonlinear sigma model with a Calabi-Yau target space, or some relatively classical background, possibly with orb folding. But as long as the central charge of the Virasoro algebra is right, and world sheet super conformal symmetry is respected, any other super conformal field theory will do just as well. Why hasn’t string theorists looked at such models more often?