Longitudinal metallicity gradient, where [Fe/H] derived from high-resolution spectroscopic observations (Genovali et al. 2014) is plotted against the Galactocentric azimuth θ. The different spiral segments are color-coded as in Fig. 7, and the size of the data-points correlates with the age of the Cepheids (larger points for larger ages). As in previous figures, only Cepheids with an age ≤ 150 Myr are considered.