  Three hyperfine states of Bose-condensed sodium atoms, recently optically
trapped, can be described as a spin-1 Bose gas. We study the behaviour of this
system in a magnetic field, and construct the phase diagram, where the
temperature of the Bose condensation $T_{BEC}$ increases with magnetic field.
In particular the system is ferromagnetic below $T_{BEC}$ and the magnetization
is proportional to the condensate fraction in a vanishing magnetic field.
Second derivatives of the magnetisation with regard to temperature or magnetic
field are discontinuous along the phase boundary.
