Metallicity effects on classical Cepheids: Velocity curve morphology of outer disc Cepheids

We present new radial velocity data for eleven classical Cepheids situated in the outer parts of the Galactic disc. The resultant velocity curves for these metal-deficient Cepheids are decomposed in Fourier parameters, and compared to solar-metallicity Cepheid data, in order to study the effect of metallicity on the pulsation properties. Up to 8 days, the phase shift is found to follow very closely the corresponding sequence for solar-metallicity Cepheids, indicating the absence of metallicity effects on for low periods. However, metal-deficient Cepheids show a slightly larger A1 amplitude and R21 amplitude ratio for days. At days, there is some evidence that the phase shift is significantly larger for metal-deficient Cepheids than at solar metallicity. A posteriori, this effect is also detected in photometric data for outer disc and LMC Cepheids in the 12-20 day period range. In good qualitative agreement with the predictions of theoretical pulsation models, we therefore see some indication of a metallicity dependence of the resonance properties. The largest effects on velocity curve shape are expected in the crucial 8-12 day period range -around the resonance for fundamental-mode Cepheids -that is not covered by our data. Possible targets for future measurements are suggested to fill this gap, and to improve the quantitative determination of metallicity effects on the structural properties of Cepheid velocity curves.