Position in the Hertzsprung–Russell diagram of OB-stars with a confirmed magnetic field detection (Briquet et al. 2013; Petit et al. 2013; Alecian et al. 2014; Fossati et al. 2014, 2015; Neiner et al. 2014) and Brott et al. (2011) evolutionary tracks. Stars with an Of?p spectral classification are highlighted with black dots. Other peculiar O-stars are indicated by green squares. HD 54879 is labelled and shown with an additional red cross. Blue triangles indicate B stars classified as peculiar in the works cited. An isochrone close to the age of HD 54879, 4.0 Myr, is plotted (purple dashed line). The averaged uncertainties in effective temperature and luminosity are illustrated by the black cross in the bottom part of the plot. The magnetic star in the Trifid nebula detected by Hubrig et al. (2014b) is not included here because of the difficulties in establishing its properties. τ Sco has also been labelled and marked (blue cross).