  The multiple star system of delta Orionis is one of the closest examples of a
system containing a luminous O-type, bright giant star (component Aa1). It is
often used as a spectral-type standard and has the highest observed X-ray flux
of any hot-star binary. The main component Aa1 is orbited by two lower mass
stars, faint Aa2 in a 5.7 day eclipsing binary, and Ab, an astrometric
companion with an estimated period of 346 years. Generally the flux from all
three stars is recorded in ground-based spectroscopy, and the spectral
decomposition of the components has proved difficult. Here we present HST/STIS
ultraviolet spectroscopy of delta Ori A that provides us with spatially
separated spectra of Aa and Ab for the first time. We measured radial
velocities for Aa1 and Ab in two observations made near the velocity extrema of
Aa1. We show tentative evidence for the detection of the Aa2 component in
cross-correlation functions of the observed and model spectra. We discuss the
appearance of the UV spectra of Aa1 and Ab with reference to model spectra.
Both stars have similar effective temperatures, but Ab is fainter and is a
rapid rotator. The results will help in the interpretation of ground-based
spectroscopy and in understanding the physical and evolutionary parameters of
these massive stars.
