  We report the determination of abundances and isotopic ratios for C, O and Si
in the photosphere of the red giant component of the recurrent nova T Coronae
Borealis from new 2.284--2.402 $\mu$m and 3.985--4.155 $\mu$m spectroscopy.
Abundances and isotopic ratios in the photosphere may be affected by (i)
processes in the red giant interior which are brought to the surface during
dredge-up, (ii) contamination of the red giant, either during the common
envelope phase of the binary evolution or by material synthesised in recurrent
nova eruptions, or a combination of the two. We find that the abundances of C,
O and Si are reasonably consistent with the expected composition of a red giant
after first dredge-up, as is the $^{16}$O/$^{17}$O ratio. The
$^{28}$Si/$^{29}$Si ratio is found to be $8.6\pm3.0$, and that for
$^{28}$Si/$^{30}$Si is $21.5\pm3.0$. The $^{12}$C/$^{13}$C ratio ($10\pm2$) is
somewhat lower than expected for first dredge-up. The $^{16}$O/$^{18}$O ratio
($41\pm3$) is highly inconsistent with that expected either from red giant
evolution ($\sim550$) or from contamination of the red giant by the products of
a nova thermonuclear runaway. In particular the C and O isotopic ratios taken
in combination are a puzzle. We urge confirmation of our results using
spectroscopy at high resolution. We also encourage a thorough theoretical study
of the effects on the secondary star in a recurrent nova system of
contamination by ejecta having anomalous abundances and isotopic ratios.
