Blue stragglers ( BSs ) are important objects in cluster populations because of their peculiar properties .
The colours and magnitudes of these objects are critical parameters in population synthesis of the host cluster and may depend remarkably on BSs ’ surface composition .
Observations show that some BSs are short-orbital-period binaries , which may be accounted for by mass transfer in low-mass binaries .
We therefore 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 .

We examine thermohaline mixing in a low-mass accreting-main-sequence star and find that , except the redistribution of composition under the surface , the mixing affects the accretor very little during Roch lobe overflow unless thermohline mixing is treated as an instantaneous process .
A series of calculations are then carried out for low-mass binaries under different assumptions .
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 Hertzsprung-Russell diagram and colour-magnitude diagram .
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 .
This is consistent with the results of Monte-Carlo simulations by previous studies .

Our simulation of F190 shows that the primary ’ s mass M _ { 1 i } of the appropriate models is located in the range of 1.40 to 1.45 M _ { \odot } with initial mass ratio q _ { i } = 1.5 and initial orbital period P _ { 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 .