5 Conclusions

We have analysed the velocity curve morphology of a set of 11 metal-deficient Cepheids situated in the outer disc of the Galaxy. The data were acquired in the context of a programme to derive surface-brightness distances to metal-poor Cepheids. The radial velocity curves were decomposed in Fourier components, and the progression of the coefficients with period compared to the solar-neighbourhood data.

We show that in the period interval 3-8 days covered by our sample, there is no significant dependence of the $\phi _{21}$ parameter on metallicity. On the contrary, our sample very precisely follows the period-$\phi _{21}$ relation defined for solar metallicity. There are however some effects of metallicity on velocity curve morphology. For P<5 days, the A₁ amplitude and R₂₁ amplitude ratio are slightly higher and the minimum period decreases markedly, for lower metallicity Cepheids.

However, photometric data and model predictions indicate that most of the effect of metallicity deficiency is likely to occur for P>8 days. Our only object in that range is HW Pup, which shows a $\phi _{21}$ phase shift as much as 1 radian above the solar metallicity counterpart. A similar difference is observed for the photometric $\phi _{21}$ for outer disc Cepheids with P>12 days, as well as for LMC Cepheids. This indicates that even for a moderate metallicity deficiency (as compared to solar), the $\phi _{21}$ progression may be rather metallicity sensitive. This effect seems to saturate for metallicities lower than LMC. However complementary data for P=8-15 days are needed before any definitive conclusion can be reached.

Acknowledgements

F. P. gratefully acknowledges support from Fondecyt postdoctoral grant 3000056 and from the Astronomy Department of Universidad de Chile, F. K. and F. P. from the Swiss National Fund for Scientific Research. W. G. is very grateful to Michel Mayor for the allocation of observing time for this program at the La Silla Swiss telescope. He also acknowledges financial support received by Fondecyt project No. 8000002. We warmly thank P. Moskalik for sending us his Fourier parameters prior to publication. We made use of the McMaster Cepheid Photometry and Radial Velocity Data Archive (dogwood.physics.mcmaster.ca/Cepheid/HomePage.html).