Einstein ring: Gravitational lensing is predicted by Albert Einstein's theory of General relativity:

In observational astronomy an Einstein ring is the deformation of the light from a source (such as a galaxy or star) into a ring through gravitational lensing of the source's light by an object with an extremely large mass (such as another galaxy, or a black hole). This occurs when the source, lens and observer are all aligned. The first complete Einstein ring, designated B1938+666, was discovered by collaboration between astronomers at the University of Manchester and Nasa's Hubble Space Telescope in 1998.[Gravitational lensing is predicted by Albert Einstein's theory of General relativity. Instead of light from a source traveling in a straight line (in three dimensions), it is bent by the presence of a massive body, which distorts spacetime. An Einstein Ring is a special case of gravitational lensing, caused by the exact alignment of the source, lens and observer. This results in a symmetry around the lens, causing a ring-like structure.[…]
Recent works about Einstein ring:

• Einstein Rings -- Astronomers Discover Eight New Gravitational Lenses with Hubble Telescope: Images from the Hubble telescope reveal eight new Einstein rings, joining only three others previously known. Einstein rings are pairs of galaxies, with a foreground galaxy bending the light of a background galaxy into a ring by gravitational effects. The ring helps astronomers precisely estimate the mass of the foreground galaxy.[1]
• Hubble Finds Double Einstein Ring: The NASA/ESA Hubble Space Telescope has revealed a never-before-seen optical alignment in space: a pair of glowing rings, one nestled inside the other like a bull's-eye pattern. The double-ring pattern is caused by the complex bending of light from two distant galaxies strung directly behind a foreground massive galaxy, like three beads on a string.[2]
• Hubble, Sloan Quadruple Number Of Known Optical Einstein Rings: Astronomers have combined two powerful astronomical assets, the Sloan Digital Sky Survey (SDSS) and NASA's Hubble Space Telescope, to identify 19 new "gravitationally lensed" galaxies. Among these 19, they have found eight new so-called "Einstein rings," which are perhaps the most elegant manifestation of the lensing phenomenon. Only three such rings had previously been seen in visible light.[3]