  In this paper we study U(1) gauge transformations on the space-time
coordinates and on the background fields $g_{\mu\nu}$ and $\phi$. For some
special gauge functions, gauged coordinates and gauged U(1) field are
equivalent to the rotated coordinates and rotated gauge field. We find gauge
transformations that are symmetries of the string action. Also we obtain
general $T$-duality transformations for the background fields. For special
background fields this duality is equivalent to a gauge transformation.
