  The $O(d,d)$ invariant worldsheet theory for bosonic string theory with $d$
abelian isometries is employed to compute the beta functions and Weyl anomaly
at one-loop. We show that vanishing of the Weyl anomaly coefficients implies
the equations of motion of the Maharana-Schwarz action. We give a
self-contained introduction into the required techniques, including beta
functions, the Weyl anomaly for two-dimensional sigma models and the background
field method. This sets the stage for a sequel to this paper on generalizations
to higher loops and $\alpha'$ corrections.
