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The properties of the interacting, eccentric orbit binary V1507 Cyg(HD187399) are examined with spectra that cover wavelengths from 0.63 to0.68um. The spectrum of the brightest star is very similar to that of the B8 Istar Beta Ori, although with absorption lines that show sub-structureconsistent with a varying tidal field. The bulk of the Halpha emission in thespectrum appears to be associated with the brighter star. Evidence is presentedthat the period of the system has been stable over timescales of many decades,arguing against large-scale mass transfer at the current epoch. Absorption andemission lines are identified that originate in an expanding asymmetricenvelope around the companion, and component masses of 6.4 +/- 0.9 and 14.0 +/-0.9 solar are found, where the former applies to the brighter star and aninclination of 46 degrees has been assumed. The evolutionary state of V1507 Cygis then that of a system in which mass transfer has progressed to the pointwhere the mass ratio has been reversed, with the brighter star stripped of muchof its initial mass. It is argued that the brighter star is an Alpha Cygvariable, and that it is those light variations that dominate the system lightcurve. V1507 Cyg is observed at or near the center of a diffuse HI bubble thatis detected at 408 and 1420MHz. It is suggested that the eccentric orbit is theresult of evolution in a hierarchical system.

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