Stacked spectrum using all exposures in all grisms. The blue section represents the grism 18 observation, the black spectrum the weighted mean of the 40 grism 19 spectra, and the red section represents the grism 20 spectrum. The positions of emission lines from H I, He I, and He II are indicated with vertical lines at the top. Some of the hydrogen lines in the grism 20 spectrum are only tentatively detected. We also mark the position of NaD absorption, which is presumably interstellar in nature. The spectrum has been normalised to 1 at 6000 Å.