We show that the significantly different effective temperatures ( \mathit { T } _ { eff } ) achieved by the luminous blue variable AG Carinae during the consecutive visual minima of 1985–1990 ( \mathit { T } _ { eff } \simeq 22 , 800 K ) and 2000–2001 ( \mathit { T } _ { eff } \simeq 17 , 000 K ) place the star on different sides of the bistability limit , which occurs in line-driven stellar winds around \mathit { T } _ { eff } \sim 21 , 000 K. Decisive evidence is provided by huge changes in the optical depth of the Lyman continuum in the inner wind as \mathit { T } _ { eff } changes during the S Dor cycle .
These changes cause different Fe ionization structures in the inner wind .
The bistability mechanism is also related to the different wind parameters during visual minima : the wind terminal velocity was 2–3 times higher and the mass-loss rate roughly two times smaller in 1985–1990 than in 2000–2003 .
We obtain a projected rotational velocity of 220 \pm 50 { km s ^ { -1 } } during 1985–1990 which , combined with the high luminosity ( \mathit { L } _ { \star } = 1.5 \times 10 ^ { 6 } \mathit { L } _ { \odot } ) , puts AG Car extremely close to the Eddington limit modified by rotation ( \Omega \Gamma limit ) : for an inclination angle of 90 ^ { \circ } , \Gamma _ { \Omega } \gtrsim 1.0 for M \lesssim 60 ~ { } \mathit { M } _ { \odot } .
Based on evolutionary models and mass budget , we obtain an initial mass of \sim 100 ~ { } \mathit { M } _ { \odot } and a current mass of \sim 60 - 70 ~ { } \mathit { M } _ { \odot } for AG Car .
Therefore , AG Car is close to , if not at , the \Omega \Gamma limit during visual minimum .
Assuming M = 70 ~ { } \mathit { M } _ { \odot } , we find that \Gamma _ { \Omega } decreases from 0.93 to 0.72 as AG Car expands toward visual maximum , suggesting that the star is not above the Eddington limit during maximum phases .