???The definitive version is available at www.blackwell-synergy.com???. Copyright Blackwell Publishing DOI: 10.1111/j.1365-2966.2008.13642.

Context. The recurrent nova RS Ophiuchi undergoes nova eruptions every ∼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 < 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 μm to determine metallicity and effective temperature of the red giant. Results. We find T eff = 4100 ± 100 K, log g = 0.0 ± 0.5, [Fe/H] = 0.0 ± 0.5, [C/H] = −0.8 ± 0.2, [N/H] = +0.6 ± 0.3 in the atmosphere of the secondary, and demonstrate that inclusion of some dust "veiling" in the spectra cannot improve our fits.

Theoretical spectral energy distributions were computed for a grid of hydrogen-deficient and carbon-rich model atmospheres of T eff in the range of 5000 − 6250 K and log g = 1.0 − 0.0 by the technique of opacity sampling, taking into account continuous, molecular band and atomic line absorption. These energy distributions were compared with the spectrum of V4334 Sgr (Sakurai's object) of April, 1997 in the wavelength interval 300 − 1000 nm. We show that (1) the shape of the theoretical spectra depends strongly on T eff , but only very weakly on the hydrogen abundance; (2) the comparison of the observed and computed spectra permits to estimate T eff ≈ 5500 K for V4334 Sgr in April, 1997, and its interstellar reddening (plus a possible circumstellar contribution) E B−V ≈ 0.70.

We compute opacities for the electronic molecular band systems A 6 + -X 6 + of CrH and CrD, and A 2 -X 2 + of MgH and MgD. The opacities are computed by making use of existing spectroscopic constants for MgH and CrH. These constants are adjusted for the different reduced masses of MgD and CrD. Frank-Condon factors are used to provide intensities for the individual vibronic bands. These results are used in the computation of synthetic spectra between T eff = 1800 and 1200 K with an emphasis on the realization of 'deuterium test', first proposed by Bejar et al. to distinguish brown dwarfs from planetary mass objects. We discuss the possible use of CrD and MgD electronic bands for the 'deuterium test'. We find CrD to be the more promising of the two deuterides, potentially, the most useful bands of CrH/CrD are the v = +1 and v = −1 at 0.795 and 0.968 μm.