V505 Mon -An early-type binary with a disk

New spectra of the eclipsing binary V505 Mon (usually classied as B5 Ib) were obtained in the optical and UV regions. Only spectral lines of one component are visible, with K =9 3 km s 1 .A ccording to arguments based on the extent of the disk the mass ratio might be about 0.30. The star with the visible spectral lines would then have a mass of 2.3, the other component of 7.7 M, i.e. their supergiant nature can be ruled out. The distance to the system appears to be about 1 kpc. Together with published photometric data, the spectroscopy provides clear evidence for the presence of a disk around the secondary component. The proles of the C ii 1335/6 doublet lines are used to derive the rotational velocity of the disk. The structure of the disk can be characterized by a central ionized zone surrounded by neutral outer parts. Additional matter of cloudy nature has to be present, and both components are embedded in an extended atmosphere. Abundance of carbon was found to be very low. Similarity with some other non-eclipsing binaries is discussed.