  We investigate radial oscillations of pure neutron stars and hybrid stars,
employing equations of state of nuclear matter from Brueckner-Hartree-Fock
theory, and of quark matter from the Dyson-Schwinger quark model, performing a
Gibbs construction for the mixed phase in hybrid stars. We calculate the
eigenfrequencies and corresponding oscillation functions. Our results for the
zero points of the first-order radial oscillation frequencies give the maximum
mass of stable neutron stars, consistent with the common criterion
$dM/d\rho_c=0$. Possible observations of the radial oscillation frequencies
could help to learn more about the equation of state, predict the maximum mass
of neutron stars more precisely, and indicate the presence of quark matter.
