  SXP 15.6 is a recently established Be star X-ray binary system (BeXRB) in the
Small Magellanic Cloud (SMC). Like many such systems the variable X-ray
emission is driven by the underlying behaviour of the mass donor Be star. It is
shown here that the neutron star in this system is exceptionally close to spin
equilibrium averaged over several years, with the angular momentum gain from
mass transfer being almost exactly balanced by radiative losses. This makes SXP
15.6 exceptional compared to all other known members of its class in the SMC,
all of whom exhibit much higher spin period changes. In this paper we report on
X-ray observations of the brightest known outburst from this system. These
observations are supported by contemporaneous optical and radio observations,
as well as several years of historical data.
