  We examine the properties and symmetries of the lowest order effective theory
of 4-dim string backgrounds with axion and dilaton fields and zero cosmological
constant. The dimensional reduction yields an O(2,2) current group of
transformations in the presence of two commuting Killing symmetries. Special
emphasis is placed on the identification of the T and S string duality
symmetries, and their intertwining relations. (Contributed to the Proceedings
of the Satellite Colloquium "Topology, Strings and Integrable Models" to the
XIth International Congress of Mathematical Physics, Paris, 25--28 July 1994;
Diderot Editeur.)
