T_eff and log g_c estimates for the Galactic (red circles) and LMC (gray triangles) O-type dwarfs as a function of SpT; corresponding linear fits are overplotted with red and black dashed lines. Stars with v sin i > 250 km s^-1 are highlighted in black. Luminosity class IV stars are plotted as red and gray open symbols and were not used for the linear fits (see text). The widely used SpT – T_eff calibration by Martins et al. (2005) is also shown for reference (red dotted line).