V Sagittae

V Sagittae or V Sge is a cataclysmic variable in the constellation Sagitta. The system is composed of a main sequence star of about 3.3 solar masses and a Wolf-Rayet star^[3] of about 0.9 solar masses.^[7] V Sge is the only super soft X-ray source nonmagnetic cataclysmic variable found so far.

A visual band light curve for V Sagittae, adapted from Šimon and Mattei (1999)^[8]

V Sagittae

Location of V Sagittae (circled) in Sagitta
Observation data Epoch J2000      Equinox J2000
Constellation	Sagitta
Right ascension	20h 20m 14.691s[1]
Declination	+21° 06′ 10.44″[1]
Apparent magnitude (V)	8.6-13.9[2]
Characteristics
Evolutionary stage	Wolf-Rayet star (A)[3] Main sequence (B)[3]
Spectral type	B1[4]
Variable type	eclipsing and cataclysmic[2]
Astrometry
Proper motion (μ)	RA: −2.133[1] mas/yr Dec.: −6.489[1] mas/yr
Parallax (π)	0.3310±0.0206 mas[1]
Distance	9,900 ± 600 ly (3,000 ± 200 pc)
Absolute magnitude (MV)	−2.2[5]
Orbit[6]
Semi-major axis (a)	4.36 R☉
Inclination (i)	71°
Semi-amplitude (K1) (primary)	320 km/s
Semi-amplitude (K2) (secondary)	85 km/s
Details
Wolf-Rayet primary[3] (V Sge A)
Mass	0.9[6] M☉
Radius	1.2[6] R☉
Luminosity	30,000[6] L☉
Temperature	70,000[6] K
Donor[3] (V Sge B)
Mass	3.3[6] M☉
Radius	2.1[6] R☉
Temperature	12,000[6] K
Other designations
AAVSO 1015+20, V Sge, GSC 01643-01764
Database references
SIMBAD	data

Material from the larger star is accreting onto the WR star at an exponentially increasing rate, generating a huge stellar wind. The doubling time for the accretion rate, and hence for the system luminosity, is about 89±11 years.^[5] It is predicted that the system will erupt as a nova some time between 2067 and 2099, at which point it will become one of the brightest stars in the sky.^[7][9]

Component A was long thought to be a white dwarf, but the primary component show similarities with a Wolf-Rayet stars and the model with a white dwarf and its accretion disk does not explain many aspects of the system, including orbital period variations and mass loss from the primary.^[3]