EW variation over time. The light curve during the MEG and LETGS observations from Kaastra et al. (2004) is plotted in the background as orange points. The vertical grey dotted lines and blue lowercase letters mark the split LETGS spectra discussed in the text. The blue points are the EW values. The dashed lines represent the average values from Table 3: light blue lines for the MEG and LETGS observations, and purple lines for the LETGS observation only.