  We formulate sigma-model duality transformations in terms of spin connection.
This allows to investigate the symmetry of the string action including higher
order $\alpha'$ corrections. An important feature of the new duality
transformations is a simple homogeneous transformation rule of the spin
connection (with torsion) and specifically adjusted transformation of the
Yang-Mills field. We have found that under certain conditions this duality is a
symmetry of the full effective string action in the target space, free of
$\alpha'$ corrections. We demonstrate how the exact duality generates new
fundamental string solutions from supersymmetric string waves.
