Very low-metallicity temperature scale for O- and early-B stars, including this paper’s results and the stars analysed by Tramper et al. (2011) and Herrero et al. (2012) in IC 1613. Different symbol size indicates different luminosity class (smallest symbols = class V, largest symbols = class I). For comparison, we have included derived temperatures for OB-stars in the SMC from Mokiem et al. (2007) and Massey et al. (2009) (violet), and Trundle et al. (2007)’s calibration for SMC B supergiants (dashed-thick) and dwarfs (dashed-thin). Solid lines represent linear regressions to the joint sample of O-giants and supergiants in the SMC (violet) and IC 1613 (blue). The high end of the temperature scale in IC 1613 is not well constrained, hence marked with a dotted line.