  The small separation (delta nu 01, delta nu 02 and delta nu 13) between the
oscillations with low degree l is dependent primarily on the sound speed
profile within the stellar core, where nuclear evolution occurs. The detection
of such oscillations for a star offers a very good opportunity to determine the
stage of its nuclear evolution, and hence its age. In this context, we
investigate the Sun and alpha Cen A and B. For alpha Cen A and B, each of the
small separations delta nu 01, delta nu 02 and delta nu 13 gives a different
age. Therefore, in our fitting process, we also employ the second difference,
defined as nu n2 - 2 nu n1 + nu n0, which is 2 delta nu 01- delta nu 02. In
addition to this, we also use frequency ratio (nu n0/ nu n2). For the Sun,
these expressions areequivalent and give an age of about 4.9-5.0 Gyr. For alpha
Cen A and B, however, the small separation and the second difference give very
different ages. This conflict may be solved by the detection of oscillation
frequencies that can be measured much more precisely than the current
frequencies. When we fit the models to the observations, we find (i) Z 0=0.020,
t=3.50 Gyr and M B=1.006 Msun from the small separations delta nu 01, delta nu
02 and delta nu 13 of alpha Cen B; and (ii) a variety of solutions from the
non-seismic constraints and delta nu 02 of alpha Cen A and B, in which the
masses of alpha Cen A and B are slightly modified and the age of the system is
about 5.2-5.3 Gyr. For Z=0.025, the closest masses we find to the observed
masses are M B=0.922 Msun and M A=1.115 Msun.The differences between these
masses and the corresponding observed masses are about 0.01 Msun.
