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MCA-1B (also called UIT003) is a luminous hot star in the western outskirtsof M33, classified over 20yr ago with a spectral type of Ofpe/WN9 andidentified then as a candidate luminous blue variable (LBV). Palomar TransientFactory data reveal that this star brightened in 2010, with a light curveresembling that of the classic LBV star AFAnd in M31. Other Ofpe/WN9 stars haveerupted as LBVs, but MCA-1B was unusual because it remained hot. It showed aWN-type spectrum throughout its eruption, whereas LBVs usually get much cooler.MCA-1B showed an almost four-fold increase in bolometric luminosity and adoubling of its radius, but its temperature stayed around 29kK. As it faded, itshifted to even hotter temperatures, exhibiting a WN7/WN8-type spectrum, anddoubling its wind speed. MCA-1B is reminiscent of some supernova impostors, andits location resembles the isolated environment of SN 2009ip. It is mostsimilar to HD5980 (in the SMC) and GR 290 (also in M33). Whereas these two LBVsexhibited B-type spectra in eruption, MCA-1B is the first clear case where aWolf-Rayet (WR) spectrum persisted at all times. Together, MCA-1B, HD 5980, andGR 290 constitute a class of WN-type LBVs, distinct from S Doradus LBVs. Theyare most interesting in the context of LBVs at low metallicity, a possiblepost-LBV/WR transition in binaries, and as likely Type~Ibn supernovaprogenitors.

A new and unusual LBV-like outburst from a Wolf-Rayet star in the outskirts of M33