Magnetic flux density, B = B_s, required for the electrons to achieve magnetic coupling in ultra-cool objects. The effect of Lyman-continuum irradiation is compared to the effect of thermal ionisation (the solid line depicts log(g) = 3.0, and the dotted line shows log(g) = 5.0). Top: brown dwarfs irradiated from an O3 star from a distance of 0.5 pc in a star-forming region (asterisks – late-M dwarfs and young brown dwarf atmospheres, log(g) = 3.0; the triangle – old brown dwarfs, log(g) = 5.0). Bottom: binary system with a WD star irradiating a brown dwarf from a distance of d = 0.006 AU (blue triangle – cold, old brown dwarf, red triangle – warm, old brown dwarf).