Observations and data reductions

The data were obtained with the 1.40m CAT telecope at La Silla ESO Observatory, using the Coudé Echelle Spectrograph (CES). The telescope was equipped with a CCD camera made of 2688x512 pixels, each having a size of 15x15$\mu$m².

Observations were made during the night November 20, 1997. The spectral domain was centered on 3933Å i.e., around the CaiiK line. The resolving power was 60,000. The integration time was 5min or less i.e., smaller than 2.5% of the pulsation period, providing a signal-to-noise ratio that we measured to be between 200 and 250 at a 2$\sigma$ level. The total coverage of these observations was a little more than 4 hours i.e., during 1.2 pulsation cycle. Our high spectral and temporal resolution observations are to be compared with DLS, who had a resolving power around 16,000 together with an integration time of about 9min i.e., 4.4% of the pulsation period. In addition, note that the time interval between consecutive spectra is about 12min for DLS, while it is about 5.6min for our observations.

Each spectrum was corrected for the pixel-to-pixel response through division by the mean of 5 flat field spectra obtained using a W-lamp. The wavelength calibration was done using about 40 lines of a thorium lamp. Then, the spectra were normalized to the continuum by a cubic spline function, and computed in the heliocentric frame.

To provide the variations as a function of phase, we used the basic period of the star i.e., P=0.1408814d (see PaperI), and the phase origin given by HIPPARCOS, which is $T_{0} = 2\,444\,995.91$d. We ensured that the derived pulsation phases were coherent with the ones obtained by DLS.