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We report the results of spectrophotometric observations of the massive starMN18 revealed via discovery of a bipolar nebula around it with the SpitzerSpace Telescope. Using the optical spectrum obtained with the Southern AfricanLarge Telescope, we classify this star as B1 Ia. The evolved status of MN18 issupported by the detection of nitrogen overabundance in the nebula, whichimplies that it is composed of processed material ejected by the star. Weanalysed the spectrum of MN18 by using the code CMFGEN, obtaining a stellareffective temperature of \approx 21 kK. The star is highly reddened,E(B-V)\approx 2 mag. Adopting an absolute visual magnitude of M_V=-6.8\pm0.5(typical of B1 supergiants), MN18 has a luminosity of log L/Lsun \approx5.42\pm0.30, a mass-loss rate of \approx (2.8-4.5)\times10^{-7} Msun/yr, andresides at a distance of \approx 5.6^{+1.5} _{-1.2} kpc. We discuss the originof the nebula around MN18 and compare it with similar nebulae produced by otherblue supergiants in the Galaxy (Sher 25, HD 168625, [SBW2007] 1) and the LargeMagellanic Cloud (Sk-69 202). The nitrogen abundances in these nebulae implythat blue supergiants can produce them from the main sequence stage up to thepre-supernova stage. We also present a K-band spectrum of the candidateluminous blue variable MN56 (encircled by a ring-like nebula) and report thediscovery of an OB star at \approx 17 arcsec from MN18. The possible membershipof MN18 and the OB star of the star cluster Lynga 3 is discussed.

The blue supergiant MN18 and its bipolar circumstellar nebula