Top: Balmer emission lines at quadrature. Two TRES spectra taken close to quadrature (red and blue lines) and one spectrum close to the system’s barycentric velocity (grey line). The grey dotted lines indicate the rest-frame wavelengths of the Balmer H_α and carbon C II 6572 lines. Orbital motion of the narrow-lined star is apparent from wavelength shifts in the C II lines, whereas the H_α line shows variable emission line profiles. Bottom: Short-term variability of emission line profiles. Three TRES spectra observed at t = MJD 60235, t + 5 d, and t + 9 d, that is, spanning < 5% of the orbital period. Solid grey, red, and blue lines show the variable H_α emission line profiles. The rest-frame wavelength of H_α is shown as in the top panel. Narrow lines at 6574, 6577 Å originate from variable telluric absorption.