As we noted in the Introduction, NGC 6397 is known to harbor a
large number of blue stragglers. We examined in detail light curves of
54 candidates for blue stragglers selected based on CMD presented in
Fig. 2 (click here).
Specifically, we selected
stars with:
a) 14.0<V<V 15.6 and (B-V)<0.6 or
b) 15.6<V< 16.0 and located to the blue of the line connecting points
and 
.
This sample included 2 earlier detected SX Phe stars.
For 47 stars we could rule out any periodic variability
with periods P<3 hours and the full amplitude exceeding
about 0.04 magnitude. Hence, the relative frequency of SX Phe stars among
NGC 6397 blue stragglers is higher or equal to 2/49. Note that some
candidates for blue stragglers can in fact be field stars, which would
increase the estimated relative frequency of occurrence of SX Phe stars.
Moreover, the example of V11 shows that some SX Phe stars can exhibit
light curves with very small amplitudes.