Title:Combined Spectroscopic and Interferometric (NPOI) Observation of the Be star o Cas. P. Koubsky, C. Hummel, P. Harmanec, R. Zavala, C. Tycner and S. Yang Abstract: The analysis of radial velocities of the Be star o Cas from spectra taken between 1992 and 2008 at Ondrejov Observatory and Dominion Astrophysical Observatory allowed us to reconfirm the binary nature of this object, first suggested by Abt and Levy in 1978, but later refuted by several authors. A sinusoidal RV curve, defined by both the H alpha emission and the He I 6678 absorption lines with K=20.0 km/s and P=1031 days, implies a very hig mass function of about 0.9 solar masses. 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 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 covering two thirds of the orbital period lead to the detection of a secondary, about 3 mag fainter than the primary. The possible properties of this peculiar binary system and the reasons why the massive secondary does not dominate the optical spectrum will be discussed.