Infrared spectrum of WRAY 15-751 from data collected at different epochs: IRAS LRS from 8 to 22 μm (1983, magenta), ISOPHOT from 2.5 to 11 μm and ISO LWS from 45 to 140 μm (1996, green), Herschel-PACS from 55 to 180 μm (2010, blue). The ISOPHOT data at λ < 5 μm are corrected for extinction using E(B − V) = 1.8. At λ > 100 μm, the flux density from ISO LWS is not reliable, the LWS aperture is too small for the ISO PSF. Color-corrected photometric measurements are superimposed (red symbols). The spectrum at λ < 5 μm comes from the central star, the bump at ~10 μm from silicates and very small out-of-equilibrium dust grains, and the spectrum at λ > 20 μm from the bulk of the dust shell. Results of the 2-Dust model fitting are illustrated. Top: best fits of the IRAS/ISO data assuming R_⋆/T_eff = 80/18 000, a_min/a_max = 0.05/1.5 (solid line) and a_min/a_max = 0.2/0.5 (dashed line). Bottom: fits of the AKARI/Herschel data using the same dust shell properties but with R_⋆/T_eff = 320/9000. Results for a lower-luminosity star R_⋆/T_eff = 240/9000 and a_min/a_max = 0.2/0.5 are also displayed (dotted line).