An infrared and visual study of the structure and variability of the cataclysmic binary V2051 Ophiuchi

This paper presents the first infrared (JHK) light curves of the ultra-short-period dwarf nova V2051 Oph in quiescence. Much of the light curve morphology can be attributed to the presence of roughly sinusoidally shaped humps, which appear at different phases in the light curves. The humps consist of material lying close to the white dwarf, that is optically thin in the visual and infrared continua, and is at least as hot as the material in the bulk of the disc. Rapid flares superposed on the humps are optically thick in the infrared, and have a visual to infrared colour temperature of ∼8000K. The luminous material in the accretion disc has an infrared brightness temperature exceeding 10 000 K, and so thermal instabilities cannot give rise to the outbursts of this object. The disc consists largely of optically thin material if the red dwarf is a normal main-sequence star. It can consist of opaque gas only if the mass-losing star is a brown dwarf with an infrared colour temperature of ∼2000 K. If such a brown dwarf is present in V2051 Oph, this binary would have a distance of ≲110 pc.