MODULI AND KAHLER POTENTIAL IN FERMIONIC STRINGS

We study the problem of identifying the moduli fields in fermionic fou r-dimensional string models. We deform a free-fermionic model by intro ducing exactly marginal operators in the form of Abelian Thirring inte ractions on the world sheet, and show that their couplings correspond to the untwisted moduli fields. We study the consequences of this meth od for simple free-fermionic models which correspond to Z(2) x Z(2) or bifolds and obtain their moduli space and Kahler potential by symmetry arguments and by direct calculation of string scattering amplitudes. We then generalize our analysis to more complicated fermionic structur es which arise in constructions of realistic models corresponding to a symmetric orbifolds, and obtain the moduli space and Kahler potential for this case. Finally we extend our analysis to the untwisted matter sector and derive expressions for the full Kahler potential to be used in phenomenological applications, and the target space duality transf ormations of the corresponding untwisted matter fields.