  In a high-mass X-ray binary (HMXB) a massive star interacts with a
neutron-star or black-hole companion in various ways. The gravitational
interaction enables the measurement of fundamental parameters such as the mass
of both binary components, providing important constraints on the evolutionary
history of the system, the equation of state of matter at supra-nuclear
density, and the supernova mechanism. The stellar wind of the massive star is
intercepted by the strong gravitational field of the compact companion, giving
rise to the production of X-rays. The X-rays increase the degree of ionization
in a small to very extended region of the surrounding stellar wind, depending
on the X-ray luminosity. This has observable consequences for the structure and
dynamics of the accretion flow. In this paper we concentrate on the fundamental
paramaters of the most massive HMXBs, i.e. those with an OB supergiant
companion, including some systems exhibiting relativistic jets
(``microquasars'').
