Comparison of a spherical shell model with the observed SED and N-band visibilities of FG Sge. The model is characterized with T_in = 1100 K, a piecewise power-law density profile with r^−3.5 (r_in ≤ r ≤ r_tr with r_in = 33 R_★ and r_tr = 5 r_in) and r⁻² (r > r_tr), τ_V = 8, and T_eff = 5000 K. a) Observed SED. Orange filled triangles correspond to visible photometric data from Arkhipova et al. (2009). Orange filled diamonds are the infrared photometric data from Shenavrin et al. (2011). The photometric data were dereddened with A_V = 0.63. The thick solid red line is the Spitzer/IRS spectrum. The thin solid blue line corresponds to the MIDI spectrum. The blue cross corresponds to VISIR photometry. The DUSTY model is shown with the solid black line. b) Observed visibilities are shown in the same manner as in Fig. 1a. The solid red and blue lines represent the model visibilities predicted for the 43.0 and 46.0 m baselines, respectively.