Multiband observations of recurrent novae

Recurrent novae are binary systems which undergo outbursts due to a thermonuclear runaway on a timescale of decades. Two systems, U Scorpii and RS Ophiuchi, are the main focus of this thesis having gone into outburst in 2010 and 2006 respectively. Optical and near-IR spectroscopy of U Sco obtained with facilities including the Liverpool Telescope, the Very Large Telescope, and the Southern African Large Telescope, as well as ultra-violet and optical spectroscopy of RS Oph obtained with the Ultra-Violet and Optical Telescope, are presented here. Observations of U Sco obtained in outburst are used to determine the helium abundance of the ejecta, with observations in quiescence used to constrain the nature of the companion star and the accretion disc. Observations of RS Oph are used to produce lightcurves of the decline from maximum to the post-outburst minimum. Analysis of optical and near-IR spectra following the 2010 outburst of U Sco leads to the determination of the reddening and the helium abundance. The reddening is found to be E(B-V) = 0.17 ± 0.14, consistent with previous estimates. The helium abundance, N(He)/N(H), is determined to be 0.122 ± 0.018, lower than previous estimates and suggesting that the companion is not helium rich. The fitting of model stellar atmospheres to observations of U Sco in quiescence, obtained ∼18-30 months after the 2010 outburst, constrains the effective temperature of the companion and indicates that it is a cool F or hot G star. The accretion disc is found to be still in a state of development until at least ∼18 months after outburst, contrary to previous studies. The strength of the Heii 4686 ̊A line in quiescence is found to be consistent with a high mass white dwarf accreting solar-abundance material at a high rate. The He line strengths do not require an overabundance of helium, as has been previously suggested, consistent with the solar abundance of helium found in the ejecta of the 2010 outburst. UVOT observations of RS Oph following the 2006 outburst are used to produce B and u′ lightcurves of the decline to minimum. These lightcurves are used in con- junction with published V and B lightcurves to show the change in the colour of the system throughout the outburst. Spectra of RS Oph show the spectroscopic evolution during the initial decline, plateau, final decline, and post-outburst minimum phases. SALT spectra obtained during the late decline phase of the 2011 outburst of a third recurrent nova, T Pyxidis, are used to derive a helium abundance of N(He)/N(H)= 0.161 ± 0.013, approximately twice the solar value. Very strong [O iii] lines are also detected.