Dependence of X-ray luminosity on the stellar luminosity for subdwarf stars. Individual symbols refer to the X-ray detected binary (filled blue circles) and single (empty blue circles) subdwarf stars and available upper X-ray detection limit in binary (filled blue triangles) and single (empty blue triangles) subdwarf stars from Tables 2 and 3. Overplotted is the extrapolation of the observed mean relation for O stars (Nazé 2009, solid red line) and predicted wind kinetic energy lost per unit of time (for Z = Z_⊙, red plus symbols).