  We study the effects of heavy quarks on the equation of state for cold and
dense quark matter obtained from perturbative QCD, yielding observables
parametrized only by the renormalization scale. We investigate the
thermodynamics of charm quark matter under the constraints of $\beta$
equilibrium and electric charge neutrality in a region of densities where
perturbative QCD is, in principle, much more reliable. We also analyze the
stability of charm stars, which might be realized as a new branch of ultradense
hybrid compact stars, and find that such quark stars are unstable under radial
oscillations.
