Milky Way metallicity gradient from Gaia DR2 F/1O double-mode Cepheids
  The ratio of the first overtone (1O) / fundamental (F) periods of mixed-mode
Cepheids that pulsate simultaneously in these two modes (F/1O) is
metallicity-dependent. It can therefore be used to characterize the systems
that host such variable stars. We want to take advantage of the F/1O
double-mode Cepheids listed in the Gaia DR2 catalogue to derive the metallicity
gradient in the Milky Way disk. The metallicity is derived from the ratio of
the first overtone and fundamental periods provided by Gaia DR2 while the Gaia
DR2 parallaxes are used to determine the Galactocentric distances of the stars.
From a visual inspection of the light curves, it turns out that a large
fraction (77%) of the Galactic F/1O double-mode Cepheids in Gaia DR2 are
spurious detections. Gaia DR2 provides 3 new bona fide F/1O Cepheids. Combining
them with the currently known F/1O Cepheids and using the Gaia DR2 parallaxes
for the entire sample, we can derive the metallicity gradient in the Milky Way
disk. We find a slope of -0.045$\pm$0.007 dex/kpc using a bootstrap method, and
of -0.040$\pm$0.002 dex/kpc using a total least squares method. These results
are in good agreement with previous determinations of the [Fe/H] gradient in
the disk based on canonical Cepheids. The period ratio of F/1O Cepheids allows
for a reliable determination of the metallicity gradient in the Milky Way, and
in turn, in other systems that would be difficult to reach via classical
spectroscopic methods.
