  We report new calculations of the physical properties of a quark-diquark
plasma. A vacuum contribution is taken into account and is responsible for the
appearance of a stable state at zero pressure and at a baryon density of about
2.2 times the nuclear matter density in this model. The resulting equation of
state was used to integrate numerically the Tolman-Oppenheimer-Volkoff
equations. The mass-radius relationship has been derived from a series of
equilibrium configurations constituted by a mixture of quarks and diquarks.
These stellar models, which are representative of a whole class, may be helpful
to understand the possible compactness of the X-ray source Her X-1 and related
objects.
