A long term spectroscopic and photometric study of the old nova HR Delphini

Context. The Nova HR Del, discovered in 1967, was found to be exceptionally bright in the optical and UV during the whole lifetime of the IUE satellite (ending in 1996) and appears to still be extremely luminous today. The reason for this continuing activity is not clear, but continuing weak thermonuclear burning might be involved. Aims. We therefore need to better understand the processes at play in HR Del. Methods. HR Del was thus monitored over several years, both in broad band photometry and spectroscopically in the Hα spectral region. Results. The profile of the Hα line shows two components: a narrow, central component and broader wings. The former is most easily understood as the result of an accretion disk whose geometry might lead it to partly occult certain regions of itself. That component shows something like an S wave with an orbital phase dependence, suggesting that it could be due to a spot bright in Hα. The wide component must come from another region, with a probably non-negligible contribution from the material ejected during the 1967 outburst. Non-orbital variations of the Hα equivalent width were found both on long and short time scales. Similar variations were found in the photometry, showing a component with a clear dependence on the orbital phase, however with no obvious relation with the Hα variations. Conclusions. The orbital part of the photometric variations can be explained by irradiation of the companion, while the properties of Hα are explicable by the presence of an accretion disk and a spot bright in Hα.