V505 Mon is an early type eclipsing binary (usually classified as B5 Ib), in which only spectral lines of the primary component are visible. The radial velocity curve provides f(m) = 4.6 M⊙, suggesting that the secondary, invisible component is more massive. We have found evidence that this star is hidden from view by the disk detected during the primary eclipse. The structure of the disk can be characterized by a central ionization zone surrounded by the neutral outer parts. The profiles of the CII 1335/6 doublet lines were used to derive the rotational velocity of the disk as well as the velocity of the stream from the primary component to the disk. The masses of the components, deduced from the fractional size of the disk, are M
1 = 1 M
⊙ and M
2 = 6.1 M
⊙. Due to the low mass of the primary component its supergiant nature can be ruled out. Additional matter of a cloudy nature is present in the system and both components are embedded in an extended atmosphere. V505 Mon seems to be a prototype of the whole group of similar early-type binaries, in which the more massive component is hidden by a circumstellar medium, most likely a disk. Probable members are V742 Cas, V1362 Cyg, V2174 Cyg, V447 Sct and FY Vel with orbital periods between 33 and 225 days. Emission in Ha is always present, but there are no emissions in IUE spectra, so these binaries are distinct from the W Ser type. The evolutionary state of the members of this group is not yet understood.