The stars in the upper left-hand region of the H-R diagram (hot stars, Wolf-Rayet stars, planetary-nebula nuclei, A-type supergiants) all have strong stellar winds that on the one hand play a major role in their evolution, and on the other constitute an important element for the chemical evolution of the Galaxy. It is widely believed that these winds are driven by radiation pressure on the gas through the many UV and EUV lines present in their spectra, and realistic models of steady-state radiation-driven winds have been produced. However, the profiles of lines formed in these winds are highly variable, and many aspects of this variability are not yet understood. In particular, we do not have a unique explanation for the narrow absorption components that appear in the UV line profiles of early-type supergiants. The study of the variability of the narrow absorption components is of utmost importance for determining their origin. A continuous monitoring of UV line profiles in these hot stars is thus necessary for studying these winds in detail. The UV Fourier spectrometer of STARS can monitor the profiles of the MgII resonance lines (A-type supergiants) and possibly of the NIV 171.8nm line (WN Wolf-Rayet and O-type stars), and thus improve our understanding of the details of the mechanisms responsible for these intense stellar winds. These luminous stars might be susceptible also to seismological studies, as the variability observed might be due to normal-mode excitation.