  This study on presolar grains compares high-precision isotopic compositions
of individual SiC grains with low 12C/13C ratios, low 14N/15N ratios, large
30Si excesses and high 26Al/27Al ratios, available in the presolar grain
database, to new CO nova models with white dwarf (WD) masses from
0.6-1.35M_sol. The models were designed to match the Large Binocular Telescope
(LBT) high dispersion spectra acquired for nova V5668 Sgr (Starrfield et al.
2018). These CO nova models provide elemental abundances up to calcium and
include mixing of WD material into the accreted material in a binary star
system under several scenarios, including one where mixing occurs only after
temperatures >7x10^7K are achieved during a thermonuclear runaway (TNR). The
0.8-1.35M_sol simulations where 25% of the WD core matter mixes with 75% of the
accreted material (assumed solar) from its binary companion after the TNR has
begun, provide the best fits to the measured isotopic data in four presolar
grains. One grain matches the 50% accreted 50% solar 1.35M_sol simulation. For
these five presolar grains, less than 25% of solar system material is required
to be mixed with the CO nova ejecta to account for the grains' compositions.
Thus, our study reports evidence of pure CO nova ejecta material in meteorites.
Finally, we speculate that SiC grains can form in the winds of cool and dense
CO novae, where the criterion C>O may not be locally imposed, and thus nova
winds can be chemically inhomogeneous.
