  We report on new measurements of the beam asymmetry for \omega\
photo-production on proton and neutron in Hydrogen and Deuterium targets from
the GRAAL collaboration. The beam asymmetry values are extracted from the
reaction threshold (E_{\gamma} = 1.1 GeV in the free nucleon kinematics) up to
1.5 GeV of incoming photon energy. For the first time both the radiative and
the three-pion decay channels are simultaneously investigated on the free
proton. Results from the two decay channels are in agreement and provide
important constraints for the determination of resonant state contributions to
the \omega\ production mechanism. First experimental results on the deuteron
allow the extraction of the \Sigma\ beam asymmetry on quasi-free nucleons.
Comparison of the results for free and quasi-free kinematics on the proton
shows a generally reasonable agreement, similar to the findings in
pseudo-scalar meson photo-production reactions. For the first time measurements
on quasi-free neutrons are available, showing that both the strength and the
angular distributions of the beam asymmetry are sensibly different from the
results on the proton target.

Nuclear Experiment