  We perform stellar evolution calculations of the remnant of the merger of two
He white dwarfs (WDs). Our initial conditions are taken from hydrodynamic
simulations of double WD mergers and the viscous disc phase that follows. We
evolve these objects from shortly after the merger into their core He-burning
phase, when they appear as hot subdwarf stars. We use our models to quantify
the amount of H that survives the merger, finding that it is difficult for
$\gtrsim 10^{-4}\;{\rm M}_\odot$ of H to survive, with even less being
concentrated in the surface layers of the object. We also study the rotational
evolution of these merger remnants. We find that mass loss over the $\sim
10^4\;\rm yr$ following the merger can significantly reduce the angular
momentum of these objects. As hot subdwarfs, our models have moderate surface
rotation velocities of $30-100\; {\rm km\,s^{-1}}$. The properties of our
models are not representative of many apparently-isolated hot subdwarfs,
suggesting that those objects may form via other channels or that our modelling
is incomplete. However, a sub-population of hot subdwarfs are moderate-to-rapid
rotators and/or have He-rich atmospheres. Our models help to connect the
observed properties of these objects to their progenitor systems.
