RU Ursae Minoris

RU Ursae Minoris is a binary star system in the constellation Ursa Minor. Its apparent magnitude ranges from 10 to 10.66 over 0.52 days as one star passes in front of the other relative to observers on Earth.^[3] Its component stars were calculated to be a primary star of spectral type F0IV/V and a secondary of spectral type K5V, both slightly more luminous than their spectral types indicate. The system is semidetached, as the secondary star is filling its Roche lobe and transferring matter to the primary.^[6] The primary is between 2.2 and 2.3 times as massive as the Sun, with 1.8 times its radius and around 8 times its luminosity. The secondary has around 0.72 times the Sun's mass, 1.1 times its radius and between 0.58 and 0.86 times its luminosity.^[7]

RU Ursae Minoris

A light curve for RU Ursae Minoris, plotted from TESS data[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Minor
Right ascension	13h 38m 56.8159s[2]
Declination	+69° 48′ 11.1694″[2]
Apparent magnitude (V)	10 - 10.66[3]
Characteristics
Spectral type	F0 IV/V + K5V
Astrometry
Proper motion (μ)	RA: 17.253±0.056[2] mas/yr Dec.: −5.004±0.049[2] mas/yr
Parallax (π)	3.5007±0.0317 mas[2]
Distance	932 ± 8 ly (286 ± 3 pc)
Absolute magnitude (MV)	2.45 ± 0.12 (A) 5.88 ± 0.19 (B)[4]
Absolute bolometric magnitude (Mbol)	2.48 ± 0.12 (A) 5.37 ± 0.19 (B)[4]
Orbit[4]
Period (P)	0.52492555(1) d
Semi-major axis (a)	3.99 ± 0.02 M☉
Inclination (i)	82.33 ± 0.05°
Periastron epoch (T)	JD 2441596.33645(6)
Details[4]
RU UMi A
Mass	2.32 ± 0.07 M☉
Radius	1.78 ± 0.02 R☉
Luminosity	7.63 ± 0.87 L☉
Surface gravity (log g)	4.30 ± 0.02 cgs
Temperature	7200 ± 200 K
RU UMi B
Mass	0.76 ± 0.02 M☉
Radius	1.14 ± 0.02 R☉
Luminosity	0.54 ± 0.10 L☉
Surface gravity (log g)	4.20 ± 0.02 cgs
Temperature	4630 ± 200 K
Other designations
BD+70°751, Gaia DR2 1686621699950649344, TYC 4402-504-1, 2MASS J13385680+6948111[5]
Database references
SIMBAD	data

The period the two take to orbit each other is decreasing very slowly (by approximately 0.15 seconds per year), suggesting the components are moving closer and will become a contact binary.^[7]