  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.
