  We studied the effects of surface composition and thermohaline mixing caused
by secular instability on the accreting components for low--mass binaries and
applied the results on a short-orbital-period BS F190 in the old cluster M67.
The results indicate no distinction in surface composition between the models
with and without thermohaline mixing during Roche lobe overflow, but we still
see the divergences of evolutionary tracks on HRD and CMD. The change of
surface composition makes the gainer bluer and smaller than the ones with
original surface composition while thermohaline mixing lessens the effect
slightly. If thermohaline mixing were to act instantaneously, the effect would
be lessened more. Our calculation shows that case A and case B mass transfer
may produce BSs in short- or relatively short-orbital-period binaries
(including Algol systems), and that CNO abundance abnormalities could be
observed in these products. Our simulation of F190 shows that the primary's
mass $M_{\rm 1i}$ of the appropriate models is located in the range of 1.40 to
1.45$M_{\odot}$ with initial mass ratio $q_{\rm i}=1.5$ and initial orbital
period $P_{\rm i}=0.8$ days, indicating that case A is a more likely
evolutionary channel than case B to form this object. The simulation also shows
that it is very likely that F190 is still in a slow stage of mass transfer. As
a consequence, obvious CNO abundance abnormalities should be observed for the
object.
