We show that in a two-dimensional sigma-model whose fields only depend on one
target space co-ordinate, the O(d,d) invariance of the conformal invariance
conditions observed at one loop is preserved at two loops (in the general case
with torsion) and at three loops (in the case without torsion).Comment: 21 pages. Plain Tex. Uses Harvmac ("b" option). Revised Version with
references added and minor errors correcte
