Luminous Blue Variable stars (LBVs) are extreme supergiants generally thought to represent a short-lived intermediate stage between O and WR stars. About ten LBVs are presently known in our Galaxy. Most of them are surrounded by a dusty nebula apparently constituted of nuclear processed material ejected by the star. The nebulae were detected by IRAS, allowing the evaluation of the dust mass in the nebula which was found to correlate with the central star luminosity, as do the ionized gas mass. Such a "nebular dust mass - stellar luminosity" relation constitutes an important constraint for understanding the mass-loss mechanisms, and for distinguishing between the various instability models. Most of these mechanisms depend on metallicity. It is therefore particularly interesting to study the properties of these objects, and especially their dusty nebulae, in other galaxies with namely different metallicities. Here, using the better sensivity provided by ISO, we propose to study the dust properties of luminous early-type stars (LBVs and the related Of/WN stars) in the Magellanic Clouds with the following questions in mind: Are the extragalactic LBVs also associated with dusty nebulae? In which proportion? Have the Of/WN stars similar dust properties? Does a "nebular mass - stellar luminosity" relation also exist? Is it quantitatively different from the galactic one? Does it depend on metallicity? Is the gas-to-dust ratio similar to the ratio found for galactic LBV-type nebulae? This can be done by measuring the continuum flux with ISOPHOT (PHT03) in the 12, 25, 60 and 100 micron IRAS bands. %Note added in PDEC: %Following the recommandations by OTAC about the feasibility, %the measurements at 60 and 100 micron with the P detector %in chopped mode (PHT03) will be done with the C detector %in raster mode (PHT22, concatenated).