V1191 Cygni

A visual band light curve for V1191 Cygni, adapted from Ostadnezhad et al. (2014)[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Cygnus
Right ascension	20h 16m 50.8045s[2]
Declination	+41° 57′ 41.360″[2]
Apparent magnitude (V)	10.99[3]
Characteristics
Spectral type	F6V + G5V[4]
Apparent magnitude (B)	11.41[3]
Apparent magnitude (R)	10.55[5]
Apparent magnitude (I)	10.06[6]
Apparent magnitude (J)	9.82[7]
Apparent magnitude (H)	9.56[7]
Apparent magnitude (K)	9.51[7]
B−V color index	0.62[8]
Variable type	W UMa[9]
Astrometry
A
Proper motion (μ)	RA: 38.4 ± 1.7[3] mas/yr Dec.: 25.1 ± 1.6[3] mas/yr
Distance	278 ± 31[10] pc
Absolute magnitude (MV)	3.82[10]
B
Absolute magnitude (MV)	5.73[10]
Orbit
Period (P)	0.3134 d[11]
Semi-major axis (a)	2.182 R☉[12]
Details
A
Mass	1.29 ± 0.08[10] M☉
Radius	1.31 ± 0.18[10] R☉
Luminosity (bolometric)	2.71 ± 0.44[10] L☉
Surface gravity (log g)	4.31[10] cgs
Temperature	6500[10] K
Age	3.85 ± 0.21[12] Gyr
B
Mass	0.13 ± 0.01[10] M☉
Radius	0.52 ± 0.15[10] R☉
Luminosity (bolometric)	0.46 ± 0.08[10] L☉
Surface gravity (log g)	4.12[10] cgs
Temperature	6610 ± 200[10] K
Age	3.85 ± 0.21[12] Gyr
Other designations
GSC 03159-01512, 2MASS J20165081+4157413, SBC9 2996, TYC 3159-1512-1
Database references
SIMBAD	data

V1191 Cygni is the variable star designation for an overcontact binary star system in the constellation Cygnus.^[13] First found to be variable in 1965, it is a W Ursae Majoris variable with a maximum apparent magnitude 10.82. It drops by 0.33 magnitudes during primary eclipses with a period of 0.3134 days, while dropping by 0.29 magnitudes during secondary eclipses.^[11] The primary star, which is also the cooler star, appears to have a spectral type of F6V, while the secondary is slightly cooler with a spectral type of G5V.^[4] With a mass of 1.29 solar masses and a luminosity of 2.71 solar luminosities, it is slightly more massive and luminous than the sun, while the secondary is only around 1/10 as massive and less than half as luminous. With a separation of 2.20 solar radii, the mass transfer of about 2×10⁻⁷ solar masses per year from the secondary to the primary is one of the highest known for a system of its type.^[10]

V1191 Cygni is a W-type W UMa variable,^[8] meaning that the primary eclipse occurs when the less-massive component is eclipsed by the larger, more massive component, although the masses are unusually different for such a system.^[14] The current period is very short for a system of its spectral type, suggesting that the stars are relatively small for their mass and age,^[14] which is likely around 3.85 billion years.^[12] The pair's orbital period is increasing at a rate of over 4×10⁻⁷ days per year, one of the fastest known rates among contact binary systems,^[8] likely due to the high rate of mass transfer. In addition to the period increase, there is cyclic period change of 0.023 days over 26.7 years, caused by either a third body with a mass of 0.77 solar masses or magnetic activity cycles. The mass transfer will likely eventually cause the system to evolve into a single star with a very high rotation rate.^[13]