m40 winnecke 4 binary / double star system in ursa major

Winnecke 4 (also known as Messier 40 or WNC 4) is a double star in the constellation Ursa Major. It was discovered by Charles Messier in 1764 while he was searching for a nebula that had been reported in the area by Johann Hevelius. Not seeing any nebulae, Messier catalogued this double star instead. It was subsequently rediscovered by Friedrich August Theodor Winnecke in 1863. Burnham calls M40 "one of the few real mistakes in the Messier catalog," faulting Messier for including it when all he saw was a double star, not a nebula of any sort.[1]

In 1991 the separation between the components was measured at 51".7, an increase since Messier's time. Data gathered by astronomers Brian Skiff (2001) and Richard L. Nugent (2002) strongly suggest that this is merely an optical double star rather than a physically connected system[2].