Measured (points) and modeled linespread function (line) for the O viii Lyα line in HR 1099 (18.97 Å), for RGS2. The assumed stellar continuum background is indicated by the lower (green) line. The modeled line is shifted by 5 mÅ, to fit the measured line position. This 5 mÅ shift is within the wavelength uncertainty and of the same order as the radial velocity shifts within the system which are between +40 and − 65 km s^-1 (Bopp & Fekel 1976), or about ± 3 mÅ. The difference between the model and the data is shown in the bottom plot.