  We investigate the properties of charge-neutral $\beta$-equilibrium cold
quark matter within the Nambu-Jona-Lasinio model. The calculations are carried
out for different ratios of coupling constants characterizing the vector and
scalar 4-fermion interaction, $\xi\equiv G_V/G_S$. It is shown that for $\xi <
0.4$ matter is self-bound and for $\xi < 0.65$ it has a first order phase
transition of the liquid-gas type. The Gibbs conditions in the mixed phase are
applied for the case of two chemical potentials associated with the baryon
number and electric charge. The characteristics of the quark stars are
calculated for $\xi = 0, 0.5$ and 1. It is found that the phase transition
leads to a strong density variation at the surface of these stars. For $\xi =
1$ the properties of quark stars show behaviors typical for neutron stars. At
$\xi\goo 0.4$ the stars near to the maximum mass have a large admixture of
strange quarks in their interiors.
