Optical/X-ray spectral energy distribution of RBS 1774. The solid line represents the updated best-fit model spectrum (kT ~ 104 eV; N_H ~ 2.6 × 10²⁰ cm^-2) to the XMM-Newton data. The arrows represent the 5σ optical/IR flux upper limits reported by Rea et al. (2007) and the 3σ near/far-ultraviolet flux upper limits derived from archival GALEX observations (see text). The VLT B-band flux of the RBS 1774 candidate counterpart is marked by the cross, while our VLT R-band upper limit is marked by the cross and the arrow. The LBT B-band measurement (Schwope et al. 2009) is marked by the triangle. The shaded areas represent the region described by the two PLs with slopes α = −2.07 and 0.07, which pass  ± 3σ above/below the B-band flux and are consistent with our R-band upper limit (see text for details). Optical points have been corrected for the N_H obtained from the updated fit to the XMM-Newton spectrum (see text). The dashed line represents the blackbody at T_O = 11 eV (r_O = 15 km, d = 150 pc), normalised to our VLT B band point and compatible with the R-band upper limit.