  Hot stars are the main source of ionization of the interstellar medium and
its enrichment due to heavy elements. Constraining the physical conditions of
their environments is crucial to understand how these stars evolve and their
impact on the evolution of galaxies. The objective of my thesis was to
investigate the physical properties of the photosphere and circumstellar
environment of massive hot stars confronting multi-band spectroscopic or
spectro-interferometric observations and sophisticated non-LTE radiative
transfer codes. My work was focused on two main lines of research. The first
concerns radiative line-driven winds. Using UV and visible spectroscopic data
and the radiative transfer code CMFGEN, I investigated the weak wind phenomenon
on a sample of nine Galactic O stars. This study shows for the first time that
the weak wind phenomenon, originally found for O dwarfs, also exists on more
evolved O stars and that future studies must evaluate its impact on the
evolution of massive stars. My other line of research concerns the study of
classical Be stars, the fastest rotators among the non-degenerated stars, and
which are surrounded by rotating equatorial disks. I studied the Be star
$\omicron$ Aquarii using H$\alpha$ (CHARA/VEGA) and Br$\gamma$ (VLTI/AMBER)
spectro-interferometric observations, the radiative transfer code HDUST, and
developing new automatic procedures to better constrain the kinematics of the
disk. This multi-band study allowed to draw the most detailed picture of this
object and its environment, to test the limits of the current generation of
radiative transfer models, and paved the way to my future work on a large
samples of Be stars observed with VEGA, AMBER, and the newly available VLTI
mid-infrared combiner MATISSE.
