Relation between observed X-ray luminosity and the bolometric luminosity for subdwarfs (adopted from Krtička et al. 2016). Blue symbols denote individual subdwarfs with X-ray detections (circles) and upper limits to X-ray luminosities (triangles). Filled symbols refer to subdwarfs in binaries, while empty symbols correspond to single stars. Black arrows with stellar identifications indicate the shift of the stellar parameters with respect to previous determinations. The solid red line extrapolates the mean observed relation for O stars (Nazé 2009).