Comparison in velocity space of line profiles of the O VII line at rest wavelength 21.6 Å (top) and an unknown absorption line observed at 26.83 Å (bottom) for the 2021d55.6 observation and the 2006d39.7, 2006d54, and 2006d66.9 observations. Assuming a similar blueshift for the unidentified line, we adopt a rest wavelength of 26.96 Å. Data are shown in histogram style and Gaussian fits to the absorption lines with solid lines. Values of best-fit blueshift are given in the bottom right legend. Observed spectra and models are normalised to the respective median values of the plot range.