Spectra of the RR Lyraes were taken with the Sandiford Echelle
Spectrometer (McCarthy et al. 1993) on the 2.1 m telescope of the McDonald
Observatory, Texas. We had four one-week runs in December 1993, March 1994,
June 1994 and December 1994. The spectra cover the wavelength range
with a resolving power of 60 000. This
wavelength range includes H
, the OI triplet at 7771, 7774 and 7775
Å and numerous lines of both calcium and iron.
The exposures times were limited to 15 minutes for the RR Lyraes in
order to prevent ``phase blurring". In general, three spectra were obtained
for each RR Lyrae. The first spectrum was arbitrarily assigned as phase zero
and then, using the published periods from the GCVS, two further spectra
were obtained, separated by plus and minus a third of a cycle from the first
spectrum. The signal-to-noise ratio of the spectra varied between 5 and 30
at H depending on the magnitude of the star and the quality of the
seeing. After each RR Lyrae observation an arc was taken.
The spectra were reduced using the MIDAS package installed at IUE, Vilspa. After bias-subtracting and flat-fielding we used the context ECHELLE within the package. This automatically finds the centre of the orders, "tracks" them across the CCD chip and then extracts them by summing over a selected input width of pixels. From tests on the order profiles we chose a width of 6 pixels which corresponds to 3 arcsec on the sky (the inter-order separation is typically 12 pixels). No background subtraction was made since tests showed this to be negligible. After wavelength calibration the centers and equivalent widths of selected lines were measured using an interactive routine within MIDAS.