V505 Mon - An early-type binary with a disk

New spectra of the eclipsing binary V505 Mon (usually classified as B5 Ib) were obtained in the optical and UV regions. Only spectral lines of one com ponent are visible, with K = 93 km s(-1). According to arguments based on t he 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 d ata, the spectroscopy provides clear evidence for the presence of a disk ar ound the secondary component. The profiles of the CII 1335/6 doublet lines are used to derive the rotational velocity of the disk. The structure of th e disk can be characterized by a central ionized zone surrounded by neutral outer parts. Additional matter of cloudy nature has to be present, and bot h components are embedded in an extended atmosphere. Abundance of carbon wa s found to be very low. Similarity with some other non-eclipsing binaries i s discussed.