Unusual radio source MG1131+0456: A possible Einstein ring

We report the discovery of extremely unusual structure in the radio source MG1131 + 0456. In a radio map with sub∼arc$econd resolution the object appears as an elliptical ring of emission accompanied by a pair of more compact sources, nearly diametrically opposed and offset ∼0.3 arc s to the south-west of the ring. There is a faint, slightly extended, optical counterpart. In its spectrum we detect a continuum but no emission lines; therefore the redshift of the counterpart is unknown. The radio morphology suggests that this source may be an example of an 'Einstein ring'¹, a highly symmetric case of grayitational lensing in which the source is imaged into a ring. Other explanations that we consider are excluded by the data. Another possibility is that MG1131 +0456 represents a new type of (unlensed) astronomical object.