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Discovery of a new Wolf-Rayet star and its ring nebula in Cygnus

We report the serendipitous discovery of a ring nebula around a candidate Wolf-Rayet (WR) star, HBHA 4202-22, in Cygnus using the Spitzer Space Telescope archival data. Our spectroscopic follow-up observations confirmed the WR nature of this star (we named it WR138a) and showed that it belongs to the WN8-9h subtype. We thereby add a new example to the known sample of late WN stars with circumstellar nebulae. We analyzed the spectrum of WR138a by using the Potsdam Wolf-Rayet (PoWR) model atmospheres, obtaining a stellar temperature of 40 kK. The stellar wind composition is dominated by helium with 20 per cent of hydrogen. The stellar spectrum is highly reddened and absorbed (E_{B-V} = 2.4 mag, A_V = 7.4 mag). Adopting a stellar luminosity of log L/Lsun = 5.3, the star has a mass-loss rate of 10^{-4.7} Msun yr^{-1}, and resides in a distance of 4.2 kpc. We measured the proper motion for WR138a and found that it is a runaway star with a peculiar velocity of ~50 km/s. Implications of the runaway nature of WR138a for constraining the mass of its progenitor star and understanding the origin of its ring nebula are discussed.