EXACTLY SOLVABLE STRING MODELS OF CURVED SPACE-TIME BACKGROUNDS

We consider a new 3-parameter class of exact 4-dimensional solutions i n closed string theory and solve the corresponding string model, deter mining the physical spectrum and the partition function. The backgroun d fields (4-metric, antisymmetric tenser, two Kaluza-Klein vector fiel ds, dilaton and modulus) generically describe axially symmetric statio nary rotating (electro)magnetic flux-tube type universes. Backgrounds of this class include both the ''dilatonic'' (a = 1) and ''Kaluza-Klei n'' (a = root 3) Melvin solutions and the uniform magnetic field solut ion, as well as some singular space-times. Solvability of the string s igma-model is related to its connection via duality to a simpler model which is a ''twisted'' product of a flat 2-space and a space dual to 2-plane. We discuss some physical properties of this model (tachyonic instabilities in the spectrum, gyromagnetic ratio, issue of singularit ies, etc.). It provides one of the first examples of a consistent solv able conformal string model with explicit D = 4 curved space-time inte rpretation.