  Based on the high sensitivity of Compton scattering off ultra relativistic
electrons, the possibility of anisotropies in the speed of light is
investigated. The result discussed in this contribution is based on the
gamma-ray beam of the ESRF's GRAAL facility (Grenoble, France) and the search
for sidereal variations in the energy of the Compton-edge photons. The absence
of oscillations yields the two-sided limit of 1.6 x 10^{-14} at 95 % confidence
level on a combination of photon and electron coefficients of the minimal
Standard Model Extension (mSME). This new constraint provides an improvement
over previous bounds by one order of magnitude.
