Context : Classical Cepheids are excellent tracers of intermediate-mass stars , since their distances can be estimated with very high accuracy .
In particular , they can be adopted to trace the chemical evolution of the Galactic disk .

Aims : Homogeneous iron abundance measurements for 33 Galactic Cepheids located in the outer disk together with accurate distance determinations based on near-infrared photometry are adopted to constrain the Galactic iron gradient beyond 10 kpc .

Methods : Iron abundances were determined using high resolution Cepheid spectra collected with three different observational instruments : ESPaDOnS @ CFHT , Narval @ TBL and FEROS @ 2.2m ESO/MPG telescope .
Cepheid distances were estimated using near-infrared ( J,H,K - band ) period-luminosity relations and data from SAAO and the 2MASS catalog .

Results : The least squares solution over the entire data set indicates that the iron gradient in the Galactic disk presents a slope of –0.052 \pm 0.003 \textrm { dex kpc } ^ { -1 } in the 5-17 kpc range .
However , the change of the iron abundance across the disk seems to be better described by a linear regime inside the solar circle and a flattening of the gradient toward the outer disk ( beyond 10 kpc ) .
In the latter region the iron gradient presents a shallower slope , i.e .
–0.012 \pm 0.014 \textrm { dex kpc } ^ { -1 } .
In the outer disk ( 10-12 kpc ) we also found that Cepheids present an increase in the spread in iron abundance .
Current evidence indicates that the spread in metallicity depends on the Galactocentric longitude .
Finally , current data do not support the hypothesis of a discontinuity in the iron gradient at Galactocentric distances of 10-12 kpc .

Conclusions : The occurrence of a spread in iron abundance as a function of the Galactocentric longitude indicates that linear radial gradients should be cautiously treated to constrain the chemical evolution across the disk .