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Our differential photometry of the eclipsing binary 68 Herculis through V- and R-filters shows periodic minima consistent with a previously established period. As a function of its orbital motion, we report spectra over a limited wavelength range encompassing H-alpha 656.3 nm and helium 667.8 nm lines. Doppler shifts of both stars were resolved in H-alpha, while only the more massive star rendered the helium line with Doppler shifts that agree with the radial velocity we derive for it from the H-alpha profile. Sinusoidal curve-fits to the orbital dependence of the radial velocities imply circular orbits for both components, with amplitudes indicating a mass ratio for the two stars in agreement with published values. A subtle Doppler shift associated with stellar rotation is evident in the radial velocity of the primary component as its eclipse commences; modeling indicates this rotation is synchronous with the orbital revolution, an expected tidal effect of near-contact binary systems.

Synchronous Rotation in the Eclipsing Binary 68 Herculis Inferred from Doppler Shifts in its Spectrum and Light Curve Modeling