  We reexamine the low-energy expansion of polarized Compton scattering off the
proton and show that the leading non-Born contribution to the beam asymmetry of
low-energy Compton scattering is given by the magnetic polarizability alone,
the electric polarizability cancels out. Based on this fact we propose to
determine the magnetic dipole polarizability of the proton from the beam
asymmetry. We also present the low-energy expansion of doubly-polarized
observables, from which the spin polarizabilities can be extracted.
