corresponding normalized flat fields, before they were rebinned 2-dimensionally to constant wavelength steps. We then filtered the uncleaned frames along the spatial axis with a median filter of 7 pixels width to erase cosmic ray hits. We identified regions uncontaminated by any stellar source and approximated the spatial distribution of the sky background by a constant. For the uncleaned frames the sky background was fit on the median filtered frames and subtracted from the unfiltered ones, whereas for the cleaned frames both sky fit and subtraction were performed on the unfiltered rebinned data. The sky-subtracted spectra were extracted using Horne’s ([@horn86]) algorithm as implemented in MIDAS. Finally the spectra were corrected for atmospheric extinction using the extinction coefficients for La Silla (Tüg [@tueg77]) as implemented in MIDAS, because they provide the closest approximation to Paranal conditions, for which no spectroscopic extinction coefficients are available.

For a relative flux calibration we used response curves derived from spectra of LTT 3864, LTT 4364, and LTT 6248 with the data of Hamuy et al. ([@hawa92]). The response curves were fit by splines and averaged for all nights. The individual target spectra were corrected for any Doppler shifts determined from Balmer and/or helium absorption 