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We discuss the evidence for the existence of bi-stable stellar winds of early type stars, both theoretically and observationally. The ratio between the terminal wind velocity and the escape velocity drops steeply from about 2.6 for stars with Teff > 21 000 K to about 1.3 at Teff < 21 000 K. This is the bi-stability jump, which is due to a change in the ionization of the wind and in the wind driving lines. The mass loss rate increases across the jump by about a factor 2 to 5 from the hotter to the cooler stars. The mass flux from rapidly rotating stars can also show the bi-stability jump at some lattitude between the pole and the equator, with a slow high density wind in the equatorial region and a faster low density wind from the poles. This might explain the disks of rapidly rotating B[e] stars, formed by the Rotation Induced Bi-stability mechanism. We discuss the RIB mechanism and its properties. We also describe some future improvements of the model.

In this review we present the ISO imaging of nebulae around Luminous Blue Variables. Three LBVs have been imaged with ISO: HR Car, AG Car and the LBV candidate G79.29+0.46. The ISOCAM instrument did not resolve the nebula around HR Car. However some nebular emission lines are seen in the spectral energy distribution of this source. A proper deconvolution of the images may resolve the nebula. For AG Car and G79.29+0.46 the nebula is clearly resolved. The structure and intensity distribution of the nebular emission is dependent on the wavelength, indicating a separation between the ionised matter and the dust in the nebulae. Some model calculations are presented for the G79.29+0.46 nebula.

We present spectra obtained with the Infared Space Observatory (ISO) of the dust shells surrounding several Luminous Blue Variables (LBVs), both in our galaxy and in the LMC. The 20-45 μm spectra of R71, AG Car and Wra 751 show prominent emission features from crystalline silicates. The composition of the crystalline silicates in LBV dust shells is compared to that found in other types of objects, such as (post)-AGB stars and red supergiants (RSG). Both Wra 751 and AG Car have a high ratio of pyroxenes to olivines. This suggests that the grains in both stars experienced very similar processing, but that this processing has lead to a higher abundance of pyroxenes compared to RSG. The dust composition of the three LBVs discussed here suggests dust formation in a cool outflow not unlike those of RSG.

A review is given of data and theories on the motion fields in super and hypergiants with special reference to LBV’s. We show that the radiative momentum flux is incapable of driving the episodical bursts of mass loss of these stars, and that there are several indications that the LBV-phenomenon is hydrodynamically driven. The sum of turbulent and radiative accelerations in the atmospheres of the most luminous stars compensates the gravitational acceleration for stars near the Humphreys-Davidson limit. This explains their atmospheric near-instability. The motion field in the atmosphere of a typical LBV consists mainly of low-order gravity waves, while acoustic waves are rapidly damped. These gravitation waves may be stochastic rather than coherently ordered. These stochastic pulsations are assumed to be responsible for the LBV phenomenon.