U Cephei is an eclipsing binary star discovered in 1880.[5][6] It consists of a blue-white main sequence star of spectral type B7/8V that is eclipsed every two and a half days by a less bright giant of type G5/8III-IV.[3] The drop in brightness lasts 4 hours and the system sees its apparent magnitude increase from 6.7 to 9.2. The total eclipse then lasts 2 hours before an increase in brightness for 4 hours. The two stars, separated by less than 10,000,000 kilometres (6,200,000 mi), exchange matter. This transfer towards the blue giant caused the system's orbital period to lengthen by 4 minutes during the 20th century. U Cephei is one of the brightest eclipsing binaries. Located near the north celestial pole, it can be monitored continuously with a 60 millimeter telescope.[7]
A light curve for U Cephei, plotted from TESS data. [8] The 2.493 day eclipse period is shown in red.
The system has two visual companions listed in the double and multiple star catalogs. U Cephei B is a twelfth magnitude star that, as of 2016, was located at an angular distance of 13.9 arcseconds and at a position angle of 63° from U Cephei A. It exhibits a common proper motion with the system, which indicates that it is physically linked to it. U Cephei C is another twelfth magnitude star, but it is only a purely optical double and its proximity to the system is a coincidence.[9]
^ abcdTupa, Peter R.; Deleo, Gary G.; McCluskey, George E.; Kondo, Yoji; Sahade, Jorge; Giménez, Alvaro; Caton, Daniel B. (2013). "Ultraviolet Spectroscopic Analysis of Transient Mass Flow Outburst in U Cephei". The Astrophysical Journal. 775 (1): 46. Bibcode:2013ApJ...775...46T. doi:10.1088/0004-637X/775/1/46.
