RW Cephei is a hypergiant variable star in the constellation Cepheus, at the edge of the Sharpless 132 HII region and close to the small open cluster Berkeley 94. One of the largest stars known, RW Cephei's radius is more than 1,000 times that of the Sun (), thus larger than the orbit of Jupiter. The temperature intermediate between the red supergiants and yellow hypergiants, and itself varying considerably, has led to it being variously considered as a red hypergiant or yellow hypergiant. Distance The distance to RW Cephei has been estimated on the basis of its spectroscopic luminosity and it is assumed to be a member of the Cepheus OB1 association. The Gaia Data Release 2 and Gaia Early Data Release 3 parallaxes lead to distance estimates of and respectively. Cepheus OB1 is generally considered to be at about . The open cluster Berkeley 94, of which RW Cephei may be a member, is thought to be at a distance of . The star and cluster are part of the larger star-forming region Sh 2-132 within the Orion-Cygnus arm of the Milky Way. Variability RW Cephei was first discovered to be variable by Henrietta Swan Leavitt in 1907. The magnitude range was initially given as 8.2–8.8 using photographic plates, while later studies found the photographic range to be from 8.6–10.7, however they note that maxima and minima cannot be derived with any certainty. Other authors estimate an amplitude of only around 0.5 magnitudes. Modern estimates put the range of variability from 6.0 to 7.6 in the V-band. The star was designated RW in 1908, being the fifteenth discovered variable in Cepheus. RW Cephei has been classified as a semi-regular variable star of type SRd, meaning that it is a slowly varying yellow giant or supergiant. The General Catalogue of Variable Stars gives a period of approximately 346 days taken from a 1952 study, but other studies suggest different periods and certainly no strong periodicity. Great dimming events In December of 2022, the star was reported by two astronomers to be going through a great dimming, reaching a fainter than usual magnitude of 7.6. It is speculated to possibly be similar to the great dimming of Betelgeuse that happened in late 2019. Previous observations using photographic plates taken between 1948 and 1951 reveal a similar dimming from magnitude 9.16 down to 9.5, followed by a rapid re-brightening to magnitude 8.9.