  With an aim of investigating the nature of evolution-induced mixing in the
envelope of evolved intermediate-mass stars, we carried out an extensive
spectroscopic study for 12 Cepheid variables of various pulsation periods
(~2-16 days) to determine the photospheric abundances of C, N, O, and Na, which
are the key elements for investigating how the H-burning products are salvaged
from the interior, based on 122 high-dispersion echelle spectra (~10 per
target) of wide wavelength coverage collected at Bohyunsan Astronomical
Observatory. Having established the relevant atmospheric parameters
corresponding to each phase spectroscopically from the equivalent widths of Fe
I and Fe II lines, we derived C, N, O, and Na abundances from C I, O I, N I,
and Na I lines by using the spectrum-synthesis fitting technique, while taking
into account the non-LTE effect. The resulting abundances of these elements for
12 program stars turned out to show remarkably small star-to-star dispersions
(<~0.1-0.2dex) without any significant dependence upon the pulsation period:
near-solar Fe ([Fe/H] ~0.0), moderately underabundant C ([C/H] -0.3),
appreciably overabundant N ([N/H] ~+0.4-0.5), and mildly supersolar Na ([Na/H]
+0.2). We conclude the following implications from these observational facts:
(1) These CNO abundance trends can be interpreted mainly as due to the
canonical dredge-up of CN-cycled material, while any significant non-canonical
deep mixing of ON-cycled gas is ruled out (though only a slight mixing may
still be possible). (2) The mild but definite overabundance of Na suggests that
the NeNa-cycle product is also dredged up. (3) The extent of mixing-induced
peculiarities in the envelope of Cepheid variables is essentially independent
on the absolute magnitude; i.e., also on the stellar mass.
