Critical mass transfer for stability. The upper red curve represents the critical mass transfer rate above which discs are hot and stable. The expected secular mean of the mass transfer is given by the black curve (details can be found in the main text). The green, magenta and blue curves represent the critical mass transfer below which discs are cold and stable (Eq. (4)), for magnetic moments of 4 × 10³², × 10³² and 2.5 × 10³¹ G cm³ respectively. The disc exists only if its inner radius is smaller than the circularisation radius; the dashed red curve shows the maximum possible mass transfer allowing the disc to be stable and cold at 1.5r_circ.