  We study the mass-radius curve of hybrid stars, assuming a single first-order
phase transition between nuclear and quark matter, with a sharp interface
between the quark matter core and nuclear matter mantle. We use a generic
parameterization of the quark matter equation of state, which has a constant,
i.e. density-independent, speed of sound ("CSS"). We argue that this
parameterization provides a framework for comparison and empirical testing of
models of quark matter. We obtain the phase diagram of possible forms of the
hybrid star mass-radius relation, where the control parameters are the
transition pressure, energy density discontinuity, and the quark matter speed
of sound. We find that this diagram is sensitive to the quark matter parameters
but fairly insensitive to details of the nuclear matter equation of state.
  We calculate the maximum hybrid star mass as a function of the parameters of
the quark matter EoS, and find that there are reasonable values of those
parameters that give rise to hybrid stars with mass above $2\,M_\odot$.
