  We study Coulomb interacting electrons confined in polygonal quantum rings.
We focus on the interplay of localization at the polygon corners and Coulomb
repulsion. Remarkably, the Coulomb repulsion allows the formation of in-gap
states, i.e. corner-localized states of electron pairs or clusters shifted to
energies that were forbidden for uncoupled electrons, but below the energies of
side-localized states. We also specify conditions allowing optical excitation
of those states.
