Upper panels: [H−K_s, J−H] and [H−K_s, K_S] diagrams of the 2MASS YSO candidates indicated by red crosses. Green asterisks correspond to main sequence stars projected onto the S 24 bubble (see Sect. 7.2). The dashed lines show the reddening vectors corresponding to M0 III and B2 V stars in the CC-diagram, and to B2 V stars in the CM-diagram. The squares are placed at intervals of ten magnitudes of visual extinction. Middle panel: location of YSO candidates from the Spitzer database. Crosses represent Class I and II sources. Bottom panel: YSO candidates from the WISE database. Class I and II sources are indicated in red and green, respectively.