  We present a study of the effect of charm quarks on the QCD equation of state
using partially-quenched p4 charm quarks on a dynamically generated 2+1 flavor
background, at zero chemical potential. We show preliminary results for the
charm quark contribution to the energy density and pressure in the high
temperature region ($T_c < T < 4 T_c$) and compare it to the free-field
calculation. The charm quark mass is determined by measuring the charmonium
spectrum.
