Introduction

Nine rings of Uranus were discovered by Earth-based stellar occultations in 1977. In order of increasing semi-major axis from the planet they have been denoted 6, 5, 4, α β η γ δ and ∈. The rings are very dark, very narrow (typically < 5 km) and have extremely sharp edges. The standard theory for the confinement of narrow rings against the spreading effects of Poynting-Robertson drag and collisions (Goldreich & Tremaine 1977) proposes that each ring is bounded by a pair of Lindblad resonances from shepherding satellites. Saturn's F-ring is now known to be shepherded by the two satellites Pandora and Prometheus, and in January 1986 Voyager 2 images of the Uranian system in back scattered light showed the presence of two satellites, Cordelia and Ophelia, on either side of the ∈ ring. Eight other small satellites, all exterior to the main rings, were discovered by Voyager 2.

Porco & Goldreich (1987) showed that the inner edge of the ∈ ring is within a kilometre of the 24:25 outer Lindblad resonance with Cordelia while the outer edge is within 300 m of the 14:13 inner Lindblad resonance with Ophelia. They identified two other possible resonances between ring features and these two satellites. A series of narrow angle Voyager 2 images of the rings were taken at fixed, non-rotating positions to search for small satellites orbiting between the rings. No further satellites were found down to a detection limit of 10 km (Smith et al. 1986).