Effective temperatures as a function of spectral type, for LMC and SMC O-stars analyzed within this work and Paper II (N11-031 and R136-040 discarded, see text). For NGC 346-355 (ON2 III(f^∗), SMC), T_eff according to our “average” solution, see text. Black symbols: LMC objects; blue symbols: SMC objects; green symbol: Sk–67° 22 (O2 If^∗/WN5, LMC). Squares refer to supergiants, triangles to (bright) giants, and circles to dwarfs. Filled symbols correspond to objects with [N]  ≥  8.0, and open ones to objects with [N]  <  8.0. The dashed-dotted (red) line displays a least square linear and quadratic fit to all dwarfs, according to Eq. (1). The T_eff calibration from Martins et al. (2005) for Galactic O-dwarfs (dashed) and supergiants (dotted) is displayed for comparison. For clarity, some objects have been slightly shifted horizontally.