Context : The recurrent nova RS Ophiuchi undergoes nova eruptions every \sim 10 - 20 years as a result of thermonuclear runaway on the surface of a white dwarf close to the Chandrasekhar limit .
Both the progress of the eruption , and its aftermath , depend on the ( poorly known ) composition of the red giant in the RS Oph system .

Aims : Our aim is to understand better the effect of the giant secondary on the recurrent nova eruption .

Methods : Synthetic spectra were computed for a grid of M-giant model atmospheres having a range of effective temperatures 3200 < \mbox { $T _ { eff } $ } < 4400 K , gravities 0 < log g < 1 and abundances -4 < [ Fe/H ] < 0.5 , and fit to infrared spectra of RS Oph as it returned to quiescence after its 2006 eruption .
We have modelled the infrared spectrum in the range 1.4 - 2.5 \mu m to determine metallicity and effective temperature of the red giant .

Results : We find \mbox { \mbox { $T _ { eff } $ } } = 4100 \pm 100 K , \log g = 0.0 \pm 0.5 , \mbox { [ Fe / H ] } = 0.0 \pm 0.5 , \mbox { [ C / H ] } = -0.8 \pm 0.2 , \mbox { [ N / H ] } = +0.6 \pm 0.3 in the atmosphere of the secondary , and demonstrate that that inclusion of some dust ‘ veiling ’ in the spectra can not improve our fits .

Conclusions :