U Cephei
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cepheus |
| Right ascension | 01h 02m 18.4440s |
| Declination | +81° 52′ 32.082″ |
| Apparent magnitude (V) | 6.92 |
| Characteristics | |
| Spectral type | B7/8V + G5/8III-IV |
| Variable type | Eclipsing binary |
| Astrometry | |
| Radial velocity (Rv) | 5.0 km/s |
| Proper motion (μ) | RA: 23.811 mas/yr Dec.: −4.381 mas/yr |
| Parallax (π) | 5.1939±0.0238 mas |
| Distance | 628 ± 3 ly (192.5 ± 0.9 pc) |
| Orbit | |
| Primary | U Cep A (B-type star) |
| Companion | U Cep B (G-type star) |
| Period (P) | 2.4928 d |
| Semi-major axis (a) | 14.7 R☉ |
| Eccentricity (e) | 0 |
| Inclination (i) | 83° |
| Semi-amplitude (K1) (primary) | 120 km/s |
| Semi-amplitude (K2) (secondary) | 180 km/s |
| Details | |
| U Cep A (B-type star) | |
| Mass | 4.2 M☉ |
| Radius | 2.9 R☉ |
| Temperature | 13600 K |
| U Cep B (G-type star) | |
| Mass | 2.8 M☉ |
| Radius | 4.7 R☉ |
| Temperature | 4950 K |
| Other designations | |
| BD+81° 25, HD 5679, HIP 4843, SAO 168 | |
| Database references | |
| SIMBAD | data |
U Cephei is an eclipsing binary star discovered in 1880. 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. 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.
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.