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Excluding supernovae, Luminous Blue Variables (LBVs) are the most variable and visually brightest stars known. They reveal an extremely peculiar pattern of photometric, spectroscopic and mass-loss variations on a wide range of time-scales. The most characteristic for the LBV phenomenon are photometric variations of $\Delta V \simeq 1$ to 2 mag, occurring on timescales of years to decades. These variations are found to occur at constant luminosity and, hence, reflect a change in bolometric correction, caused by a change of photospheric radius and temperature.
The required radius change amounts to a factor of 4 to 12, depending on the luminosity. The physical mechanism responsible is not known. However, two theories have been proposed which should at least confine the region of origin to either the atmosphere or the subphotospheric layers. In the first theory, the variations are due to a drastic, but unexplained, increase in mass loss which causes the formation of a `pseudo' photosphere in the stellar wind (Davidson 1987, ApJ 317, 760). (A star is said to have a `pseudo' photosphere if the continuum is formed at or above the sonic point) In the second theory, the variations are at least partly due to an unexplained expansion of the subphotospheric layers (Leitherer et al. 1989, ApJ 346, 919).
We present the results of an extensive parameter study using ISA-Wind Non-LTE atmospheres for stars with stellar winds (de Koter et al. 1993, A\&A 277, 561) and show how LBVs react to changes in mass loss rate and velocity structure of the wind. We explain why no significant `pseudo' photospheres form in LBV winds but under which circumstances they may appear in other types of stars. Finally, we discuss which type of photometric variations in LBVs may be connected to mass-loss variations. All our results support the theory of Leitherer et al., and we therefore conclude that the origin of the mechanism causing the $\Delta V \simeq 1$ to 2 mag variations should be located below the atmosphere.
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