Dust-to-gas mass ratios as a function of metallicity. Open circles show inferred from the Hi gas mass of SBS 0335−052 and I Zw 18; the filled (blue) circles give the total gas mass after adding the putative molecular component discussed in Sect. 5.1. Galaxies from Galametz et al. (2011) are shown as (green) open stars, those from Draine et al. (2007) as (red) +, from James et al. (2002) as (purple) squares, and from Leroy et al. (2011) as (magenta) open circles. The short-dashed line is the linear prediction by Draine et al. (2007), and the three-dot-dashed curves below show the models by Hirashita et al. (2002). The vertical dotted line illustrates solar metallicity 12 + log (O/H) = 8.69 (Asplund et al. 2009).