  A large class of cosmological solutions (of the Einstein equations) in string
theory, in the presence of Maxwell fields, is obtained by $O(d,d)$
transformations of simple backgrounds with $d$ toroidal isometries. In all the
examples in which we find a (closed) expanding universe, such that the universe
admits a smooth, complete initial value hypersurface, a naked singularity may
form only at the time when the universe collapses. The discrete symmetry group
$O(d,d,Z)$ identifies different cosmological solutions with a background
corresponding to a (relatively) simple CFT, and therefore, may be useful in
understanding the properties of naked singularities in string theory.
