Observed positions in the Gaia observational HR diagram of potential (see text) members of the σ Orionis cluster (magenta filled circles), with Gaia sources in the cluster field of view with parallaxes of better than 10% shown in small gray points in the background. The uncertainties on both photometry and parallax are shown for all cluster members, being smaller than the size of the points for the majority of them. Superposed are shown isochrones for a metallicity of Z = 0.014 from Haemmerlé et al. (2019) for different values of the logarithm of the age (in years). The positions of σ Ori C, D, and E are highlighted in the diagram as shown in the inset.