  We combine the output of hydrodynamical simulations of Population III star
cluster formation with stellar evolution models, and calculate the evolution of
protostars experiencing variable mass accretion rates due to interactions
within a massive disk. We find that the primordial protostars are extended
'fluffy' objects for the bulk of their pre-main-sequence lifetimes. Accretion
luminosity feedback from such objects is high, but as shown in previous work,
has a minimal effect on the star cluster. The extended radii of the protostars,
combined with the observation of close encounters in the simulations, suggests
that mergers will occur in such systems. Furthermore, mass transfer between
close protostellar binaries with extended radii could lead to massive tight
binaries, which are a possible progenitor of gamma ray bursts.
