  We present spectropolarimetric radio images of the supernova remnant (SNR)
G296.5+10.0 at frequencies near 1.4 GHz, observed with the Australia Telescope
Compact Array. By applying rotation measure (RM) synthesis to the data, a
pixel-by-pixel map of Faraday rotation has been produced for the entire
remnant. We find G296.5+10.0 to have a highly ordered RM structure, with mainly
positive RMs (mean RM of +28 rad/m**2) on the eastern side and negative RMs
(mean RM of -14 rad/m**2) on the western side, indicating a magnetic field
which is directed away from us on one side and toward us on the other. We
consider several possible mechanisms for creating the observed RM pattern.
Neither Faraday rotation in foreground interstellar gas nor in a homogeneous
ambient medium swept up by the SNR shell can easily explain the magnitude and
sign of the observed RM pattern. Instead, we propose that the observed RMs are
the imprint of an azimuthal magnetic field in the stellar wind of the
progenitor star. Specifically, we calculate that a swept-up magnetized wind
from a red supergiant can produce RMs of the observed magnitude, while the
azimuthal pattern of the magnetic field at large distances from the star
naturally produces the anti-symmetric RM pattern observed. Expansion into such
a wind can possibly also account for the striking bilateral symmetry of the
SNR's radio and X-ray morphologies.
