Comparison between optical and X-ray light curve of HD 167971. Top panel: V-band photometry of HD 167971 (Mayer et al. 2010) folded with the ephemerides of the inner eclipsing binary. Bottom panel: X-ray flux in the 0.5–10 ke V band corrected for the interstellar absorption and folded with the ephemerides from Ibanoglu et al. (2013). We note that the uncertainties on the ϕ_inner of the X-ray data are smaller than the size of the symbols. They range between 1.6 × 10⁻³ for the oldest data and 2.8 × 10⁻³ for the most recent observation.