  We discuss present and future cosmological constraints on variations of the
fine structure constant $\alpha$ induced by an early dark energy component
having the simplest allowed (linear) coupling to electromagnetism. We find that
current cosmological data show no variation of the fine structure constant at
recombination respect to the present-day value, with $\alpha$ / $\alpha_0$ =
0.975 \pm 0.020 at 95 % c.l., constraining the energy density in early dark
energy to $\Omega_e$ < 0.060 at 95 % c.l.. Moreover, we consider constraints on
the parameter quantifying the strength of the coupling by the scalar field. We
find that current cosmological constraints on the coupling are about 20 times
weaker than those obtainable locally (which come from Equivalence Principle
tests). However forthcoming or future missions, such as Planck Surveyor and
CMBPol, can match and possibly even surpass the sensitivity of current local
tests.
