OB stars and Luminous Blue Variables
The most direct method of deriving empirical mass-loss rates for hot stars is through analysis of the UV resonance transitions from dominant metal ions. A major advance is anticipated through the recent HST ULLYSES programme, involving high resolution STIS and COS spectroscopy of OB stars in the Magellanic Clouds, which has been followed up with VLT visible Xshooter spectroscopy: XshootU (Xshooting ULLYSES)
Binary evolution is now recognised to play a dominate role for OB stars, from previous Galactic and LMC studies, the latter using VLT/FLAMES spectroscopy of OB stars in 30 Doradus (VFTS). A recent initiative focused on multiple epoch spectroscopy of SMC OB stars VLT/FLAMES is currently underway: BLOeM
Regarding, LBVs, I have monitored a Luminous Blue Variable star in the Magellanic galaxy NGC 2366 10,000,000 light years away. This star has a B supergiant spectral type, except that it is currently undergoing a giant eruption, such that one solar earth mass of material is being ejected every day!
Such high and constant mass-loss is exceptional even amongst massive stars, although all are irregular and structured at a basic level. It is believed that such intense episodes of mass-loss are responsible for the removal of hydrogen envelopes in massive stars, causing evolution to the Wolf-Rayet phase.
More information
Physical parameters and wind properties of galactic early B supergiants, Crowther et al. 2006
Spectral evolution of the LBV NGC2363-V1, Petit et al. 2006.
The VLT-FLAMES survey of massive stars: Mass loss and rotation of early-type stars in the SMC, Mokiem et al. 2006.
The VLT-FLAMES survey of massive stars: Wind properties and evolution of hot massive stars in the LMC, Mokiem et al. 2007.
The empirical metallicity dependence of the mass-loss rate of O and early-B type stars, Mokiem et al. 2007.
Bolometric luminosity variations in the luminous blue variable AFGL2298, Clark et al. 2009.
A FEROS spectroscopic study of the extreme O supergiant He3-759, Crowther and Evans 2009
The Onfp class in the Magellanic Clouds, Walborn et al. 2010.
Spectral classification of O2-3.5If*/WN5-7 stars, Crowther and Walborn 2011.