We report on measurements of the magneto-optical properties of excitons confined in ring-shaped self-assembled semiconductor quantum dots. The rings are embedded in a field-effect structure that allows the number of confined electrons to be set electrostatically. In addition, electron-hole pairs are generated optically. The resulting photoluminescence spectra of neutral, singly and doubly charged excitons were measured at as a function of the applied magnetic field (0–). The emission energy shows a diamagnetic shift as well as a Zeeman splitting. We measured the emission of different charge states of the exciton in many individual dots. In a few of the measured rings, a new behavior was observed, namely a clear departure from the low field diamagnetic dispersion for fields larger than .