U Cephei

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.

U Cephei

U Cephei, as seen during the Digitized Sky Survey
Observation data Epoch J2000      Equinox J2000
Constellation	Cepheus
Right ascension	01h 02m 18.4440s[1]
Declination	+81° 52′ 32.082″[1]
Apparent magnitude (V)	6.92[2]
Characteristics
Spectral type	B7/8V + G5/8III-IV[3]
Variable type	Eclipsing binary
Astrometry
Radial velocity (Rv)	5.0[4] km/s
Proper motion (μ)	RA: 23.811[1] mas/yr Dec.: −4.381[1] mas/yr
Parallax (π)	5.1939±0.0238 mas[1]
Distance	628 ± 3 ly (192.5 ± 0.9 pc)
Orbit[3]
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[3]
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[2]
Database references
SIMBAD	data

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]

See also

• AR Cephei
• List of variable stars