Singlet couplings and (0,2) models

We use the quantum symmetries present in string compactification on Landau-Ginzburg orbifolds to prove the existence of a large class of exactly marginal (0,2) deformations of (2,2) superconformal theories. Analogous methods apply to the more general (0,2) models introduced earlier by the present authors, lending further credence to the fact that the corresponding Landau-Ginzburg models represent bona fide (0,2) SCFTs. We also use the large symmetry groups which arise when the worldsheet superpotential is turned off to constrain the dependence of certain correlation functions on the untwisted moduli. This allows us to approach the problem of what happens when one tries to deform away from the Landau-Ginzburg point. In particular, we find that the masses and three-point couplings of the massless E6 singlets related to H1(End(T)) vanish at all points in the quintic Kaehler moduli space. Putting these results together, and invoking some plausible dynamical assumptions about the corresponding linear σ-models, we show that one can deform these Landau-Ginzburg theories to arbitrary values of the Kahler moduli. ((orig.))