SIMBAD references

We investigate the effect of different metallicities on the light curve shape of classical double-mode (beat) Cepheids pulsating in the fundamental and first overtone modes by correlating the light curve Fourier parameters with [Fe/H] ratios. We combine data of known Galactic beat Cepheids with those from both Magellanic Clouds assembled by the OGLE-III and OGLE-IV surveys. The [Fe/H] values for the Galactic sample are based on recent high-resolution spectroscopy. For the Magellanic sample we apply the metallicity-period-period ratio relationship to estimate individual [Fe/H] values. We confirm that the metallicity is most strongly correlated with the amplitude of the light curve and the amplitude ratio of the first and second harmonics ( R₂₁), similar to single-mode classical Cepheids found by Klagyivik. We also show that the metallicity is less correlated with φ₃₁ parameter in our sample, as compared with the case of the RR Lyr type variables. We derive empirical formulae that can be used to estimate the [Fe/H] of classical Cepheids that extends well below the metallicity of the Milky Way disk, down to the lower metallicities of the Magellanic Clouds.