Double stars are of two types; optical pairs (that is to say, line-of-sight effects) and binaries (physically-associated pairs). Binaries are much the more frequent. They range from ‘contact pairs’, where the components are almost or quite touching, to very distant pairs separated by at least a light-year. In a binary system the components move round their common centre of gravity. For visual binaries, the shortest period is that of Wolf 630 Ophiuchi (1.725 years), but shorter periods are known; the record-holder is X-1820-303, an X-ray star in the globular cluster NGC 6623, distance 30 000 light-years. Its period is 685 s or 11 min. It was discovered in 1987 by the aptly-named Luigi Stella and collaborators with the Exosat satellite. It is impossible to say which is the binary with the longest period, and all we can say is that very widely-separated components share a common motion through space.