Moduli and Kaehler potential in fermionic strings

We study the problem of identifying the moduli fields in fermionic four-dimensional string models. We deform a free-fermionic model by introducing exactly marginal operators in the form of Abelian Thirring interactions on the world sheet, and show that their couplings correspond to the untwisted moduli fields. We study the consequences of this method for simple free-fermionic models which correspond to Z2xZ2 orbifolds and obtain their moduli space and Kaehler potential by symmetry arguments and by direct calculation of string scattering amplitudes. We then generalize our analysis to more complicated fermionic structures which arise in constructions of realistic models corresponding to asymmetric orbifolds, and obtain the moduli space and Kaehler potential for this case. Finally we extend our analysis to the untwisted matter sector and derive expressions for the full Kaehler potential to be used in phenomenological applications, and the target space duality transformations of the corresponding untwisted matter fields