| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cygnus |
| Right ascension | 20h 16m 50.8045s [2] |
| Declination | +41° 57′ 41.360″ [2] |
| Apparent magnitude (V) | 10.82 –11.15 [3] |
| Characteristics | |
| Spectral type | F6V + G5V [4] |
| Apparent magnitude (B) | 11.41 [5] |
| Apparent magnitude (R) | 10.55 [6] |
| Apparent magnitude (I) | 10.06 [7] |
| Apparent magnitude (J) | 9.82 [8] |
| Apparent magnitude (H) | 9.56 [8] |
| Apparent magnitude (K) | 9.51 [8] |
| B−V color index | 0.62 [9] |
| Variable type | W UMa [10] |
| Astrometry | |
| Radial velocity (Rv) | −32±13 [11] km/s |
| Proper motion (μ) | RA: +38.115 [11] mas/yr Dec.: +23.341 [11] mas/yr |
| Parallax (π) | 4.5063±0.0127 mas [11] |
| Distance | 724 ± 2 ly (221.9 ± 0.6 pc) |
| A | |
| Absolute magnitude (MV) | 3.82 [12] |
| B | |
| Absolute magnitude (MV) | 5.73 [12] |
| Orbit | |
| Period (P) | 0.3134 d [3] |
| Semi-major axis (a) | 2.182 R☉ [13] |
| Details | |
| A | |
| Mass | 1.29 ± 0.08 [12] M☉ |
| Radius | 1.31 ± 0.18 [12] R☉ |
| Luminosity (bolometric) | 2.71 ± 0.44 [12] L☉ |
| Surface gravity (log g) | 4.31 [12] cgs |
| Temperature | 6500 [12] K |
| Age | 3.85 ± 0.21 [13] Gyr |
| B | |
| Mass | 0.13 ± 0.01 [12] M☉ |
| Radius | 0.52 ± 0.15 [12] R☉ |
| Luminosity (bolometric) | 0.46 ± 0.08 [12] L☉ |
| Surface gravity (log g) | 4.12 [12] cgs |
| Temperature | 6610 ± 200 [12] K |
| Age | 3.85 ± 0.21 [13] 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. [14] 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. [3] 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. [12]
V1191 Cygni is a W-type W UMa variable, [9] 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. [15] 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, [15] which is likely around 3.85 billion years. [13] 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, [9] 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. [14]