New constraints on varying $\alpha$

We present a summary of recent constraints on the value of the fine-structure constant at the epoch of decoupling from the recent observations made by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite. Within the set of models considered, a variation of the value of $\alpha$ at decoupling with respect to the present-day value is now bounded to be smaller than 2% (6%) at 95% confidence level. We point out that the existence of an early reionization epoch as suggested by the above measurements will, when more accurate cosmic microwave background polarization data is available, lead to considerably tighter constraints. We find that the tightest possible constraint on $\alpha$ is about $0.1$ using CMB data alone--tighter constraints will require further (non-CMB) priors.