  Photospheric abundances of C, N, O, and Na were determined by applying the
synthetic spectrum-fitting technique to 34 snap-shot high-dispersion spectra of
22 RR Lyr stars covering a metallicity range of -1.8 <[Fe/H] < 0.0, with an aim
of investigating the mixing mechanism in the interior of low-mass giant stars
by examining the abundance anomalies of these elements possibly affected by the
evolution-induced dredge-up of nuclear burning products. Special attention was
paid to check the recent theoretical stellar evolution simulations indicating
the importance of thermohaline mixing in low-mass stars (M <~1 M_sun), which is
expected to be more significant as the metallicity is lowered. By inspecting
the resulting abundances in comparison with those of unevolved metal-poor
dwarfs at the same metallicity, the deficiency in C as well as enrichment in N
was confirmed (while O is almost unchanged), the extent of peculiarities
tending to increase with a decrease in [Fe/H]. Accordingly, the [C/N] ratio
turned out to progressively decrease towards lower metallicity from ~0
(Fe/H]~0) to ~-1 ([Fe/H]~-1.5), which is reasonably consistent with the
theoretical prediction in the presence of thermohaline mixing. However, these
RR Lyr stars do not show any apparent Na anomaly (i.e., essentially the same
[Na/Fe] vs. [Fe/H] trends as those of dwarfs), despite that
metallicity-dependent overabundance in Na is theoretically expected for the
case of non-canonical mixing. This inconsistency between C/N and Na may suggest
a necessity of further improvement in the current theory.
