Top panels: 2MASS J−H vs. H−K_s for objects around ρ Ophiuchi (left panel), and for L1688 (right panel). Bottom panels: H vs. H−K_s for the same two regions. Isochrones at 10 Myr (solid line) and 5 Myr (dashed line) (Siess et al. 2000) at the distance of ρ Ophiuchi (120 pc) are traced in both panels. Symbols are as in Fig. 2. Reddening vectors corresponding to A_V = 3 are indicated. A_V = 3 is close to the extinction value derived from gas absorption N_H around ρ Ophiuchi, and this is about ten times lower than in L1688.An age comprised between 5 and 10 Myr for most of objects is also inferred from the best fit of L_bol and T_eff (Sect. 3.2).