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.
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.