WR 140 (= HD 193793 = BD +43° 3571 = V1687 Cyg) is a massive binary system comprising a WC7 type Wolf-Rayet star and an O5 star, probably a supergiant. Both stars have fast (3000 km/s), radiatively driven, stellar winds carrying significant mass loss, about 10^-5 and 10^-6 solar masses/year (WC7 and O5 stars respectively), and therefore having significant kinetic power (mechanical luminosity), equivalent to 10⁴ and 10³ Solar luminosities. Between the two stars, the winds collide, releasing some of this power (about 10³ solar luminosities) and producing shocks, resulting in particle acceleration and heating and compression of the plasma in the shocked winds. The consequences of these colliding-wind processes are observed at X-ray and radio wavelengths, making WR140 one of the brightest non-compact stellar X-ray sources [1] and a non-thermal radio source [2]. Variations of the X-ray and radio emission are observed as the stars move in their eccentric binary orbit [3]. High-resolution imaging of the stars themselves has provided an astrometric orbit [6] , completing the definition of the orbit in three dimensions.

WR140 was also the first Wolf-Rayet star to show a sudden brightening in its infrared flux, attributed to an episode of dust formation [4]. This dust (a form of carbon, rather like soot) condenses in the stellar wind, absorbing a small fraction of the stars' UV-optical radiation. This radiation heats the dust to a temperature of about 1000K, causing the brightening at infrared wavelengths. The newly formed dust is dispersed by the stellar wind and cools as it is carried further from the stars and experiences less heating by their radiation, causing its infrared emission to fade. A second episode of dust formation by WR140 was observed in 1985, and it was argued that these events were periodic and also linked to a binary orbit, occurring near the time of periastron passage [3].
Similar episodes were observed in 1993, 2001 and 2009 [8], having a period near 2897 days, equal to that of the binary orbit. WR140 is the prototype periodic episodic dust-maker. The dust cloud formed in the 2001 episode has been imaged at high resolution in the infrared: in mid-2001 with aperture-masking on the Keck telescope by Monnier et al. [5], then with near-IR cameras and adaptive optics systems on the Hale and William Herschel telescopes, and then at wavelengths between 3.6 and 10 µm with UKIRT and Gemini (North) as the cloud expanded and cooled [7]. Some of the 4-µm images are shown below.

Right: K-band "light curves" near the 1977, 1985, 1993, 2001 and 2009 maxima from data in [3], [4], [7] and [8], phased to the orbit.

Far right: Infrared (4 µm) image of WR 140 observed in November 2002, (phase 0.23) showing dust emission south and east of the star (scale 4 x 4 arcseconds).

The most recent periastron passage occurred in 2016, and stimulated an intensive multi-wavelength observing campaign. Results from the 2009 periastron campaign were reported at two meetings in 2010: Stellar Winds in Interaction" and the 39th Liège International Astrophysical Colloquium "The multi-wavelength view of Hot Massive Stars" including 2009 Campaign results.

WR 140 has been observed by most X-ray missions. Recent results from SUZAKU reported by Suguwara et al. show that there is still a great deal about the system and colliding wind physics that we do not understand.

Critical configurations of WR 140 in 2016-17 and 2024-25