Fig. 6. The spectrum of S Gru (full line) is compared with two synthetic spectra based on the dynamical atmosphere p5t288solu2 at phase 0.4 (bolometric, dotted line) and a MARCS model with , , solar mass and chemical abundances (dashed line), respectively. The V = 2 0 and V = 3 1 bandheads of the main SiO isotope are shown. The synthetic spectra are calculated using a spherical radiative transfer and normalized to the same pseudo-continuum as the observations.