Quantum Symmetries of String Vacua

We show that for a large class of string vacua, by a proper tuning of the metric (and the axionic fields), one obtains global symmetries. Moreover, the string vacua which arise as orbifoldized Landau-Ginzburg models are equipped with precisely this metric. These symmetries are not symmetries of the underlying manifold, rather they are symmetries of the superconformal theory manifest only at the Quantum level. These generalize the duality symmetries well known for toroidal compactifications, to a large class of string vacua. Moreover we discuss in detail the global symmetries of the quantum K3 space, and present evidence that the global symmetries of the moduli space of this theory is SO(20, 4; Z).