  In the cores of young dense star clusters repeated stellar collisions
involving the same object can occur, which has been suggested to lead to the
formation of an intermediate-mass black hole. In order to verify this scenario
we compute the detailed evolution of the merger remnant of three sequences. We
follow the evolution until the onset of carbon burning and estimate the final
remnant mass to determine the ultimate fate of a runaway merger sequence.
  We use a detailed stellar evolution code to follow the evolution of the
collision product. At each collision, we mix the two colliding stars, taking
account of mass loss during the collision. During the stellar evolution we
apply mass loss rates from the literature, as appropriate for the evolutionary
stage of the merger remnant. We compute models for high ($Z=0.02$) and low
($Z=0.001$) metallicity to quantify metallicity effects.
  We find that the merger remnant becomes a Wolf-Rayet star before the end of
core hydrogen burning. Mass loss from stellar winds dominates over the mass
increase due to repeated mergers for all three merger sequences that we
consider. In none of our high metallicity models an intermediate-mass black
hole is formed, instead our models have a mass of 10--14 \Msun{} at the onset
of carbon burning. For low metallicity we expect the final remnant of the
merger sequence to explode as a pair creation supernova. We find that our
metal-rich models become inflated as a result of developing an extended
low-density envelope. This may increase the probability of further collisions,
but self-consistent $N$-body calculations with detailed evolution of runaway
mergers are required to verify this.
