  We examine the properties of dark energy and dark matter through the study of
the variation of the electromagnetic coupling. For concreteness, we consider
the unification model of dark energy and dark matter, the generalized Chaplygin
gas model (GCG), characterized by the equation of state
$p=-\frac{A}{\rho^\alpha}$, where $p$ is the pressure, $\rho$ is the energy
density and $A$ and $\alpha$ are positive constants. The coupling of
electromagnetism with the GCG's scalar field can give rise to such a variation.
We compare our results with experimental data, and find that the degeneracy on
parameters $\alpha$ and $A_s$, $A_s \equiv A / \rho_{ch0}^{1+\alpha}$, is
considerable.
