Interferometry and spectroscopy of beta Cen: a beta Cephei star in a binary system

beta Cen is a bright beta Cephei variable and has long been suspected to be a binary. Here we report interferometric observations with the 3.9-m Angle -Australian Telescope at a single epoch which show the star to be a binary with components separated by 15 milliarcsec and having approximately equal luminosities at 486 nm. We also present high-resolution spectra taken over five nights with the ESO CAT which show beta Cen to be a double-lined spect roscopic binary. We identify two pulsation frequencies in the primary. Furt her spectroscopic and interferometric studies of this double star should al low determination of its orbital parameters and component masses.