  We search for signatures of Lorentz and $CPT$ violations in the cosmic
microwave background (CMB) temperature and polarization anisotropies by using
the WMAP and the 2003 flight of BOOMERANG (B03) data. We note that if the
Lorentz and $CPT$ symmetries are broken by a Chern-Simons term in the effective
lagrangian, which couples the dual electromagnetic field strength tensor to an
external four-vector, the polarization vectors of propagating CMB photons will
get rotated. Using the WMAP data alone, one could put an interesting constraint
on the size of such a term. Combined with the B03 data, we found that a nonzero
rotation angle of the photons is mildly favored: $\Delta \alpha=
-6.0^{+4.0}_{-4.0}$ $^{+3.9}_{-3.7}$ deg (1, 2 $\sigma$).
