Limits on particles of small electric charge

We consider experimental limits on particles of small (not necessarily rational) electric charge. Such particles are possible within the standard model, and may be a natural consequence of extensions of the standard model incorporating an extra U(1) gauge group associated with a "mirror universe" sector. For both these cases we examine the limits from low-energy quantum electro-dynamics corrections, direct accelerator searches, stellar astrophysics, constraints on big-bang nucleosynthesis, and relic cosmological densities. The combined results exclude significant regions of the charge-mass parameter plane.