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 co mpared to solar-metallicity Cepheid data, in order to study the effect of m etallicity on the pulsation properties. Up to P similar or equal to 8 days, the phi (21) phase shift is found to follow very closely the corresponding sequence for solar-metallicity Cepheids, indicating the absence of metalli city effects on phi (21) for low periods. However, metal-deficient Cepheids show a slightly larger A(1) amplitude and R-21 amplitude ratio for P <5 da ys. At P >12 days, there is some evidence that the phi (21) 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 ag reement with the predictions of theoretical pulsation models, we therefore see some indication of a metallicity dependence of the P-2/P-0 resonance pr operties. The largest effects on velocity curve shape are expected in the c rucial 8-12 day period range - around the P-2/P-0 resonance for fundamental -mode Cepheids - that is not covered by our data. Possible targets for futu re measurements are suggested to fill this gap, and to improve the quantita tive determination of metallicity effects on the structural properties of C epheid velocity curves.