X-ray luminosity (5–40 Å, 0.3–2.5 keV) versus bubble size for the bubble sub-grid with WR composition and C = 3 × 10⁻⁷ erg cm⁻² s⁻¹ K^−7∕2, and for ages from 200 to 1000 yr. Black dots indicate individual bubble models. The slow wind, Ṁ _sw , ranges from 10⁻⁷ to 10⁻⁴ M_⊙ yr⁻¹, the slow-wind velocity, V _sw, from 10 to 40 km s⁻¹ (cf. Table 1). The adopted values for BD +30 (see text) are marked by the filled star. The open stars indicate the positions if the assumed distance to BD +30 is increased or decreased by a factor of two.