V1507 CYGNI (HD187399): A Highly Evolved, Enigmatic Interacting Binary System with an Eccentric Orbit

The properties of the interacting, eccentric orbit binary V1507 Cyg (HD187399) are examined with spectra that cover wavelengths from 0.63–0.68 μm. The spectrum of the brightest star is similar to that of the B8 I star β Ori, although with absorption lines that show substructure possibly related to a varying tidal field. The bulk of the Hα emission in the spectrum appears to be associated with this star. Evidence is presented that the period of the system has been stable over timescales of many decades, arguing against large-scale mass transfer at the current epoch. Absorption and emission lines are identified that originate in an expanding asymmetric envelope around the companion, and component masses of 6.4 ± 0.9 and 14.0 ± 0.9 M⊙ are found, where the former applies to the brighter star and an inclination of 46° has been assumed. Mass transfer has thus progressed to the point where the mass ratio has reversed. It is argued that the brighter star is an α Cyg variable, and that it is those light variations that dominate the system light curve. V1507 Cyg is observed at or near the center of a diffuse H i bubble that is detected at 408 and 1420 MHz. It is suggested that the eccentric orbit is the result of evolution in a hierarchical system, in which a now-defunct massive third body recently disturbed the orbit of the stars in V1507 Cyg, thereby disrupting mass transfer.