Luminous Blue Variables (LBVs) are the most luminous variable blue stars, showing moderate (0.5--2 mag) light changes on timescales of decades apparently at constant bolometric luminosity. LBVs are considered to be at an intermediate stage between massive O stars and W-R stars in which the outer layers are removed by extreme mass-loss. The LBV definition is very broad and includes the S Dor, P Cygni type and Hubble-Sandage variables, with around a dozen LBVs are currently known in the Galaxy and LMC and around 20 further afield. LBVs lie close to the Humphreys-Davidson instability limit (Humphreys & Davidson 1979), which is an observed luminosity cutoff above which red supergiants are not found. The LBV variability has therefore been linked to the instability which allows the most massive stars to lose sufficient mass to prevent them from becoming red supergiants although the mechanism for this instability remains poorly known.
The luminosities of LBVs () are comparable with W-R stars, while they typically resemble A supergiants (S Dor type, 7--8kK) at visual maximum, and B supergiants at minimum (P Cyg type, 15--20kK), though some stars proceed to still earlier spectral type at visual minimum (see AG Car in Table 1). A major problem relating to the discovery of new LBVs is that the timescale over which variability occurs is generally much longer than that which observations have been made (e.g. He 3--519, Smith et al. 1994).
Mass-loss rates of LBVs ( yr ) are also comparable with W-R stars, and appear to stay fairly constant throughout their evolution across the H-R diagram. Terminal wind velocities are found to be dependent on spectral type, with wind velocities increasing from around 100 km s in their A--type phase, to 250 km s for their hot phase.
Evidence for major eruptions in the past (e.g. 1600--1660 for P Cygni) are circumstellar shells and ejecta nebulae (also seen around some W-R stars), while de Koter et al (1996) have recently presented a physical parameter study for LBV photometric and spectroscopic variations.
It has been only in recent years that estimates of chemical abundances in LBVs have been possible. Standard Model analyses of P Cyg (Langer et al. 1994) and AG Car (Smith et al. 1994) revealed atmospheres highly enriched in helium (H/He2.5, by number). Chemical abundances in LBV nebulae also indicate enrichment in helium and nitrogen, supporting the idea that LBVs represent chemically evolved post-main sequence stars, although the long variability timescales inhibit a more complete understanding of LBV.