Constraints on cosmic birefringence using $E$-mode polarisation
  A birefringent universe could show itself through a rotation of the plane of
polarisation of the cosmic microwave background photons. This is usually
investigated using polarisation $B$ modes, which is degenerate with
miscalibration of the orientation of the polarimeters. Here we point out an
independent method for extracting the birefringence angle using only
temperature and $E$-mode signals. We forecast that, with an ideal
cosmic-variance-limited experiment, we could constrain a birefringence angle of
$0.3^\circ$ with $3\,\sigma$ statistical significance, which is close to the
current constraints using $B$ modes. We explore how this method is affected by
the systematic errors introduced by the polarisation efficiency. In the future,
this could provide an additional way of checking any claimed $B$-mode derived
birefringence signature.
