Relation between observed X-ray luminosity and the stellar luminosity for subdwarfs (adopted from Krtička et al. 2016). Blue symbols refer to individual subdwarfs: circles denote X-ray-detected subdwarfs, while triangles denote available upper X-ray detection limits, filled symbols denote subdwarfs in binaries, and empty symbols correspond to single objects. Overplotted is the extrapolation of the observed mean relation for O stars (Nazé 2009, solid red line). The shift of the stellar parameters with respect to previous determinations is denoted using the black arrow.