A spectroscopic study of the X-ray binary V1727 Cygni.

Time-resolved high-resolution spectroscopy is used to study the binary X-ray source V1727 Cyg (=4U2129 + 47). The He II 4686 emission line has a double-peaked line profile, a strong S-wave distortion, and distinctive eclipse behavior-features suggesting that the line is formed in an accretion disk around the compact object. H-beta absorption and a sharp emission component at N III 4640 arise on the heated face of companion star. The radial velocities of the lines, the duration of the X-ray eclipse, and the separation of the He II emission peaks constrain the masses at 0.6 + or - 0.2 solar mass for the compact star and 0.4 + or - 0.2 solar mass for the companion. If the compact component is a neutron star, as seems probable from the X-ray luminosity, its derived mass is significantly lower than previously determined neutron star masses.