  Classical Cepheids (CCs) are solid distance indicators and tracers of young
stellar populations. Our aim is to provide iron, oxygen, and sulfur abundances
for the largest and most homogeneous sample of Galactic CCs ever analyzed. The
current sample covers a wide range in Galactocentric distances (RG), pulsation
modes and periods. High-resolution and high S/N spectra collected with
different spectrographs were adopted to estimate the atmospheric parameters.
Individual distances are based on Gaia trigonometric parallaxes or on
near-infrared Period-Luminosity relations. We found that Fe and alpha-element
radial gradients based on CCs display a well-defined change in the slope for RG
larger than 12 kpc. Radial gradients based on open clusters, covering a wide
range in age, display similar trends, meaning that the flattening in the outer
disk is an intrinsic feature of the radial gradients since it is independent of
age. Empirical evidence indicates that the radial gradient for S is steeper
than for Fe. The difference in the slope is a factor of two in the linear fit.
We also found that S is, on average, under-abundant compared with O. We
performed a detailed comparison with Galactic chemical evolution models and we
found that a constant Star Formation Efficiency for RG larger than 12 kpc takes
account for the flattening in both Fe and alpha-elements. To further constrain
the impact that predicted S yields for massive stars have on radial gradients,
we adopted a "toy model" and we found that the flattening in the outermost
regions requires a decrease of a factor of four in the current S predictions.
Sulfur photospheric abundances, compared with other alpha-elements, have the
key advantage of being a volatile element. Therefore, stellar S abundances can
be directly compared with nebular S abundances in external galaxies.
