Emergent spectrum of Lyman-α photons (x_in = 0) emitted at the centre of a uniform static sphere of H I mixed with neutral deuterium D. The model parameters are similar to those used in Fig. 3 of Dijkstra et al. (2006): T = 10⁴ K, N_H I = 2 × 10¹⁹ cm⁻², and D/H = 3 × 10⁻⁵. We recover the same absorption feature as Dijkstra et al. (2006) on the blue side of the Lyman-α line (x ≈ 6.3), which is due to the presence of deuterium, compared to the case for pure H I (dashed line).