Line ratio of O VIII/O VII over line ratio Ne X/Ne IX for BD +30 (error cross) after Yu et al. (2009, Table 2) and for ZP96 heat-conducting bubbles with homogeneous hydrogen-poor WR composition subject to various absorptions as indicated in the legend. Individual models are only shown for N_H = 0 (circles) and N_H = 0.20 × 10²² cm⁻², the value adopted (grey dots). For other values of N_H, only envelopes are given in order to avoid confusion. Evolution proceeds from low to high line ratios, that is, from low to high T_X. The oldest (1000 yr) and hottest (4 MK) bubble models always have neon line ratios of about 2, virtually independent of N_H. The crosses are the models that lie within BD +30’s error box. The filled square (green) marks our “best-fit” bubble model, presented in Sect. 4.3. The filled triangles are bubbles with constant temperatures (iso-HB, for N_H = 0.20 × 10²² cm⁻² only, see text for details). The numbers along the iso-HB sequence indicate the bubble temperatures in MK.