c © ESO 2008 Astronomy &Astrophysics The decline in irradiation from the white dwarf in old novae

Aims. We have investigated how a flux ratio analysis of the light curves of cataclysmic variables can be used to calculate the luminosity irradiating the secondary star in the classical novae QU Vul, V Per, DD Cir, DN Gem, V1432 Aql, and WY Sge. Methods. We undertook phase-resolved, near-infrared K band photometry of QU Vul and V Per. Using data from QU Vul we show how flux ratios taken between fiducial orbital phases in the light curves of irradiated CVs can be used to measure the degree of heating of the secondary star. We compared the heating effect obtained from flux ratio analysis with more formal modelling, or by measurements taken from the literature, and found good agreement. We used the results to determine how irradiation changes with time since the nova outburst. Results. The light curve of QU Vul shows the presence of two maxima in the K band, which are displaced from phase 0.25 towards the 0.5 phase position, as would be expected from heating of the inner face of the secondary star by radiation from hot primary. Nova V Per, on the other hand shows evidence for a hot spot on the accretion disc, and it would appear that heating of the inner face is not occurring. The results of the flux ratio analysis of the objects examined are plotted as a function of time since the nova explosions occurred. There is marginal evidence for a decline in flux with time since the outburst, superimposed on considerable scatter, which is likely to be caused by the different temperature reached in each nova explosion. The decline is consistent with the declines others have seen. We conclude that it is the decline in reprocessed irradiation from the cooling white dwarf alone, rather than a decline in mass transfer rate, that could be the cause of the decrease in optical brightness seen in old novae