Left: colour–colour diagram for all 340 2MASS counterparts of X-ray sources found in fields E, C, S, and W. Blue symbols represent disk-bearing (Class I and II) stars classified in Sect. 4.2. The ZAMS and locus of giant stars are indicated by thick lines, while the locus of T Tauri stars is represented by a thin line. Dotted and dashed lines show reddening vectors. Right: colour–magnitude diagram showing the isochrones 0.2, 1, 5, 10, 15, 20 Myr, and ZAMS (full lines), early main sequence (grey thick line) and evolutionary pre-MS tracks 0.1, 0.5, 1, 2, 3, 4, 5, 6, and 7 M_⊙ (dashed line) from Siess et al. (2000).