Position of the underluminous sdBs (black diamonds) in the HRD (top), Kiel diagram (middle), and T_eff-log N(He)/N(H) plane. The underluminous sdBs were selected from their position below the ZAEHB for a core mass of 0.45 M_⊙. The ZAEHB and TAEHB computed with STELUM for core masses of 0.45 M_⊙ (solid lines) and 0.40 M_⊙ (dashed lines) are indicated. All stars are color-coded with the mass obtained from the SED fits. We only show the stars with spectral types that can be associated with the EHB region: sdB, iHe-sdB, and iHe-sdOB.