  We compare quark stars made of color-superconducting quark matter to
normal-conducting quark stars. We focus on the most simple
color-superconducting system, a two-flavor color superconductor, and employ the
Nambu-Jona-Lasinio (NJL) model to compute the gap parameter and the equation of
state. By varying the strength of the four-fermion coupling of the NJL model,
we study the mass and the radius of the quark star as a function of the value
of the gap parameter. If the coupling constant exceeds a critical value, the
gap parameter does not vanish even at zero density. For coupling constants
below this critical value, mass and radius of a color-superconducting quark
star change at most by ca. 20% compared to a star consisting of
normal-conducting quark matter. For coupling constants above the critical value
mass and radius may change by factors of two or more.
