  It is shown that the critical temperature for spin-triplet, p-wave
superconductivity mediated by spin fluctuations is generically much higher in a
Heisenberg ferromagnetic phase than in a paramagnetic one, due to the coupling
of the magnons to the longitudinal magnetic susceptibility. Together with the
tendency of the low-temperature ferromagnetic transition in very clean
Heisenberg magnets to be of first order, this qualitatively explains the phase
diagram recently observed in UGe_2.
