  The dynamics of entanglement and fidelity for a subsystem of two separate
spin-1/2 qubits prepared in Bell states is investigated. One of the subsystem
qubit labelled $A$ is under the influence of a Heisenberg XY spin-bath, while
another one labelled $B$ is uncoupled with that. We discuss two cases: (i) the
number of bath spins $N\to\infty$; (ii) $N$ is finite: N=40. In both cases, the
bath is initially prepared in a thermal equilibrium state. It is shown that the
time dependence of the concurrence and the fidelity of the two subsystem qubits
can be controlled by tuning the parameters of the spin-bath, such as the
anisotropic parameter, the temperature and the coupling strength with qubit
$A$. It is interesting to find the dynamics of the concurrence is independent
of four different initial Bell states and that of the fidelity is divided into
two groups.
