Observations and data reductions

The observations were carried out with the cross-dispersed spectrograph ELODIE (Baranne et al. [1996]). This instrument is at the Cassegrain focus of the 1.93m telescope of the Observatoire de Haute-Provence. The detector was a thinned Tk-CCD, with $1024 \times 1024$ elements of size 24$\mu$m².

The observations were obtained during two nights, namely August 1$^{\rm st}$ and 8$^{\rm th}$, 1994. The spectral domain ranges from 3900Å to 6800Å with a resolving power of 42000. The typical exposure time was 6min, leading to a signal-to-noise ratio around 50 for the lines considered in this paper (essentially $\lambda \lambda$4553 Siiii and H$\alpha$ lines). We reduced the spectra with the INTER-TACOS software (Baranne et al. [1996]) that takes care of the offset and flat-field pixel-to-pixel corrections. Finally, the spectra were normalized to the continuum using a cubic spline function and computed in the heliocentric frame.

Approximately, each night represents 1.2 pulsation cycle (P=4h 50mn). The pulsation phases $\varphi$ were computed using the ephemeris given by Odell ([1994]):

HJD = 2447700.1158 + 0.201044444d.