Introduction

In the pulsating stars, $\beta$ Cephei, RR Lyrae, classical Cepheids and $\delta$ Scuti, a differential variation of the radial velocity of metallic lines and Balmer ones was observed. Several explanations are proposed (Mathias & Gillet 1993). The most importants are a phase lag due to the propagation time of the wave between the different line forming regions, and a variation of the thermodynamical conditions in the line forming region.

Historically, this velocity difference was observed for the first time by Van Hoof and Struve (1953) in the $\beta$ Cephei star $\beta$ CMa. A similar lag was detected later in many $\beta$ Cephei stars such as 16 Lac (Van Hoof et al. 1954), BW Vulpeculae (Mc Namara et al. 1955), $\sigma$ Scorpii (Struve et al. 1955) and $\alpha$ Lupi (Mathias & Gillet 1993). Van Hoof effect was also observed in other pulsating star classes e.g., $\delta$ Scuti stars $\beta$ Cas (Yang et al. 1982) and $\rho$ Pup (Mathias et al. 1997), classical Cepheids (Wallerstein et al. 1992; Butler 1993) and RR Lyrae stars RR Lyrae (Mathias et al. 1995, Paper I).

Recently, the Van Hoof effect has been studied in much more details, and even detected between several metallic lines in all pulsating stars above: $\beta$ Cephei star BW Vulpeculae and $\alpha$ Lupi (Mathias & Gillet 1993), $\delta$ Scuti stars 20 CVn (Mathias & Aerts 1996) and classical Cepheids (Wallerstein et al. 1992). However, in RR Lyrae stars, no Van Hoof effect is observed between metallic lines. Mathias et al. (1995) interpreted the absence with the help of nonlinear nonadiabatic pulsational models of RR Lyrae itself (Fokin 1992). On one hand, they suggested that the absence is due to the fact that all metallic lines are formed in the same region, all relevant layers are then submitted, at approximatively the same time, to the same physical effects. On the other hand, the shock wave only forms in the high atmosphere where the hydrogen line cores are produced and not in the deep atmosphere where the metallic line cores are formed.

Recently, Chadid & Gillet (1996a, 1997) detected, for the first time, the line doubling phenomenon on two metallic absorption lines of the brighest RR Lyrae star: RR Lyrae. It was interpreted as the consequence of a ``two-step'' Schwarzchild's mechanism. A complete theoretical study was recently given by Fokin and Gillet (1997). Thus a strong shock wave propagates throughout the photospheric layers contrary to the initial Fokin's theoretical conclusion (1992). It is first receding, then stationary during a short time and finally advancing. Its amplitude varies with Blazhko phase approximately from a Mach number 4 to 6. Consequently, it is important to revise the fundamental question : does the Van Hoof effect exist between metallic lines in the RR Lyrae stars ?

The goal of this paper is devoted to the detection of this phenomenon. In Section2 we describe the observations and the data reduction process. The detection of the Van Hoof effect is performed in Sect.3 and discussed in Sect.4. Finally, the last section is devoted to some general concluding remarks.