lines. These shifts varied more than expected from one observation to the next. This can be understood by the fact that the seeing disk was smaller than the slit width, so that small shifts in the position of the stars within the slit can introduce shifts in the wavelength. A shift of 01 corresponds to a shift of 0.6 Å. Any remaining cosmic ray hits in the stellar spectra were corrected by comparing the individual spectra of the stars. Due to the seeing being smaller than the slit width spectra of the same star from different nights can have different resolution. In order to get a well defined resolution of the co-added spectra, we convolved spectra obtained with better seeing to the resolution determined by the worst resolution of each setup. This resulted in resolutions of 5.4 Å (09) for setup a, 4.8 Å (08) for setup b and 4.0 Å (07) for setup c.

[psfile=0505\_3.ps hscale=75 vscale=75 hoffset=14 voffset=-20 angle=0]{}

Analysis\[sec-analysis\]
========================

The spectra, given in Fig. \[spec\], show a large variety of helium line strengths. Part of this variation is due to variations in effective temperature, but – as we will see in the analysis – part is 