A new and unusual LBV-like outburst from a Wolf-Rayet star in the
  outskirts of M33
  MCA-1B (also called UIT003) is a luminous hot star in the western outskirts
of M33, classified over 20yr ago with a spectral type of Ofpe/WN9 and
identified then as a candidate luminous blue variable (LBV). Palomar Transient
Factory data reveal that this star brightened in 2010, with a light curve
resembling that of the classic LBV star AFAnd in M31. Other Ofpe/WN9 stars have
erupted as LBVs, but MCA-1B was unusual because it remained hot. It showed a
WN-type spectrum throughout its eruption, whereas LBVs usually get much cooler.
MCA-1B showed an almost four-fold increase in bolometric luminosity and a
doubling of its radius, but its temperature stayed around 29kK. As it faded, it
shifted to even hotter temperatures, exhibiting a WN7/WN8-type spectrum, and
doubling its wind speed. MCA-1B is reminiscent of some supernova impostors, and
its location resembles the isolated environment of SN 2009ip. It is most
similar to HD5980 (in the SMC) and GR 290 (also in M33). Whereas these two LBVs
exhibited B-type spectra in eruption, MCA-1B is the first clear case where a
Wolf-Rayet (WR) spectrum persisted at all times. Together, MCA-1B, HD 5980, and
GR 290 constitute a class of WN-type LBVs, distinct from S Doradus LBVs. They
are most interesting in the context of LBVs at low metallicity, a possible
post-LBV/WR transition in binaries, and as likely Type~Ibn supernova
progenitors.
