Observations

**Figure:** Variation of the FWHM of the unblended FeI5576.0883Å absorption line of $\delta$ Cephei. There are 288 spectra obtained over 3 years from September 1993 to October 1995. The sample standard deviation is 0.21km/s
$\begin{figure} \resizebox{\hsize}{!}{\includegraphics{fig1.ps}}\end{figure}$

To know the exact shape of the FWHM of a metallic absorption line and to detect possible fine details, it is necessary to obtain observations with the best time resolution. As discussed in our previous work (Breitfellner and Gillet 1993), we have selected in the spectrum of the classical Cepheid star $\delta$ Cephei the FeI absorption line at 5576.0883Å which is free of blends.

The observations were done at the 1.52 m telescope of the Observatoire de Haute-Provence (OHP) in September 8-20, 1993 (59 spectra), September 15-28, 1994 (65 spectra), from August 22 to September 4, 1995 (125 spectra) and from September 28 to October 2, 1995 (59 spectra) with the spectrograph AURELIE (Gillet et al. (1994). The same high resolution configuration of the spectrograph as in FGB was used. The dispersion was of 3.23Åmm^-1 (0.042Å pixel^-1 or 2.26 kms^-1pixel^-1) and led to a resolution of 0.113Å (6.1kms^-1, $R \equiv \lambda / \Delta \lambda \simeq 49\,000$ ). Integration times were between several minutes and half an hour, depending on the magnitude of $\delta$ Cephei (between 3.5 and 4.5) and the weather conditions, giving a signal-to-noise ratio around $400 \pm 100$ at the continuum level. The data reduction processing is described in Breitfellner and Gillet (1993) and we have used the same ephemeris to compute pulsation phases.

Finally, 288 FWHM have been measured between pulsation cycle number 1208 and 1438 i.e., over an interval of 230 cycles (period of 5.4 days). $\delta$ Cephei is known to have a stable average velocity curve (shape and amplitude) on time scales of a century (Butler 1993). Although our observations (Fig.

) refers to a single photospheric absorption line, it is not excluded that a no strict reproductibility from one cycle to another of the dynamical motion of the line formation atmospheric layers, over our two years of observations, induces a weak dispersion. An analysis of the distribution of FWHM measures versus phase shows that the scattering is approximately the same over the whole pulsation period. Thus, no small bumps or high frequency features within the FWHM curve are clearly present. Only a broad peak centered at phase 0.82 and a large ``stillstand'' around the luminosity maximum (phase 0.0) are detected.

Figures

and

show the residual flux (RF) and the equivalent width (EW) of the FeI line respectively while (Fig.

) gives the radial velocity curve. This latter has been measured with a Gaussian fit.

**Figure:** Same as Fig. but for the residual flux. The sample standard deviation is 0.004
$\begin{figure} \resizebox{\hsize}{!}{\includegraphics{fig2.ps}}\end{figure}$

**Figure:** Same as Fig. but for the equivalent width (EW). The sample standard deviation is 0.003Å
$\begin{figure} \resizebox{\hsize}{!}{\includegraphics{fig3.ps}}\end{figure}$

**Figure:** Same as Fig. but for the restframe radial velocity. Note that the projection effect over the stellar disc was not taken into account. The sample standard deviation is near 0.2km/s
$\begin{figure} \resizebox{\hsize}{!}{\includegraphics{fig4.ps}}\end{figure}$

**Figure:** Overplot of a fit of curves of Figs.- . The ordinate scale is arbritrary
$\begin{figure} \resizebox{\hsize}{!}{\includegraphics{fig5.ps}}\end{figure}$