  In this paper, we study cosmologies containing the reduced modified Chaplygin
gas (RMCG) fluid which is reduced from the modified Chaplygin gas
$p=A\rho-B\rho^{-\alpha}$ for the value of $\alpha=-1/2$. In this special case,
dark cosmological models can be realized for different values of model
parameter $A$. We investigate the viabilities of these dark cosmological models
by discussing the evolutions of cosmological quantities and using the currently
available cosmic observations. It is shown that the special RMCG model ($A=0$
or $A=1$) which unifies the dark matter and dark energy should be abandoned.
For $A=1/3$, RMCG which unifies the dark energy and dark radiation is the
favorite model according to the objective Akaike information criteria. In the
case of $A<0$, RMCG can achieve the features of the dynamical quintessence and
phantom models, where the evolution of the universe is not sensitive to the
variation of model parameters.
