2 Observations

The 11 programme targets were chosen from the list of 50 outer disc Cepheids in Pont et al. (1997). These are classical Cepheids in the outer disc of the Galaxy with galactocentric distances between 11 and 14 kpc (assuming R₀=8 kpc), and are moderately metal-deficient ([Fe/H $]\simeq-0.2$ to -0.4 dex).

A total of 202 spectra were obtained on the Coralie spectrometer installed on the Euler telescope at La Silla, Chile, between 25 December 1999 and 11 March 2000. Coralie is an improved copy of the Elodie instrument (Baranne et al. 1996), a high-resolution ( $R \sim 45\,000$) echelle spectrometer optimized for radial velocity determinations. Radial velocities for the Cepheid spectra were determined on-line by cross-correlation with a numerical template based on an F0 spectrum. The timing of the observations was carefully chosen in order to obtain regular phase coverage along the pulsation cycle. The exposure times were between 6 and 30 min depending on target magnitude. The uncertainties are typically in the 0.3-0.6 km$\,$s^-1 range. Simultaneous visual and infrared photometry was also obtained for the target stars on the YALO telescope at Cerro Tololo, Chile, in view of a radius and distance determination using the surface brightness technique. The radial velocity data is available on request to the first author and will be published in extenso later, along with the photometric data (to be reduced). The individual radial velocity curves are displayed in Fig. 1.

Figure 1: Radial velocity curves for our program Cepheids. Object names and periods (in days) are indicated on the left side of the plot. The period values are given in Table 1 and discussed in Sect. 3. Dots - CORALIE and ELODIE data, Circles - CORAVEL data, Triangles - Interpolated points, Crosses - Rejected points. Error bars are smaller than the symbol size.

The uncertainties computed by the Coralie on-line reduction procedure have been checked by acquiring ten spectra of TZ Mon in rapid succession on the night of 1st January 2000. The observed dispersion in the computed radial velocities was found to confirm the validity of the stated uncertainties.

The radial velocity measurements obtained with Coralie were pooled with previous measurements for the same objects published in Pont et al. (1994) and Pont et al. (1997), measurements realized with the Elodie and Coravel spectrometer on the 1.93-m telescope and Swiss telescope respectively, at Observatoire de Haute-Provence (France) between 1984 and 1997. The Elodie and Coralie radial velocity zero-points are very nearly identical, having been cross-calibrated to the level of a few meters per second in the context of planet searches (the Coralie team, priv. comm.). Coravel measurements have also been put on the Elodie/Coralie scale, as explained by Udry et al. (1999), using a transformation depending on colour and velocity. The uncertainties on this transformation are of the order of 0.1 km$\,$s^-1. Measurements uncertainties for faint Cepheids are typically 0.3-0.6 km$\,$s^-1 with the Coralie and Elodie spectrometers and 0.5-1.0 km$\,$s^-1 with Coravel.

In total, 20 to 37 radial velocity measurements are available for each of the programme stars, defining their velocity variations very accurately (see Fig. 1).