Combined spectroscopic and interferometric (NPOI) observations of the Be star o Cassiopeiae

The analysis of radial velocities of the Be star o Cas from spectra taken between 1992 and 2008 at Ondřejov and Dominion Astrophysical Observatories allowed us to reconfirm the binary nature of this object, first suggested by Abt and Levy in 1978, but later refuted by several authors. The orbital parameters of this SB1 system imply a very high mass function of about one solar mass. This in turn leads to a very high mass of the secondary, possibly higher than that of the primary. In order to check for the existence of such a massive secondary, o Cas was observed with the Navy Prototype Optical Interferometer, which allowed the binary components to be spatially resolved for the first time. The interferometric observations lead to the detection of a secondary, about 3 mag fainter than the primary. The possible properties of this peculiar binary system are discussed.