Notes. In Veselova & Nikiforov (2020), the Cepheids are split into an old (P ≤ 9 d) and a young (P ≥ 9 d) sample. Heliocentric distances of the spiral arms of Veselova & Nikiforov (2020) are taken from their Table 1, and converted into Galactocentric distances by adding 7.33 kpc and multiplying the outcome by a correction factor of 1.117, as indicated by those authors. The Galactocentric distances of the spiral arms in this study are the average of the reference radius of individual groups (see Table 4). Groups representative of a given spiral arm have been selected by comparing Fig. 8 in this study and Fig. 5 in Veselova & Nikiforov (2020). For the Cepheids groups identified by Genovali et al. (2014), we use directly the tabulated Galactocentric distances or their average value if we combine two of their groups. We note that Genovali et al. (2014) used 7.94 kpc (Groenewegen et al. 2008) as the Galactocentric distance of the Sun.