An extended red wing of the 5797 Å diffuse interstellar band in the Red Rectangle nebula

Diffuse interstellar bands (DIBs) are interstellar molecular absorption features that are ubiquitous in the interstellar medium (ISM). So far, there was only one known sight line in which DIBs show an extend tail towards the red (ETR), namely Herschel 36. Such sight lines are essential to illuminate the DIB problem, namely the identification of their carriers, because they show more DIB variation than standard ISM sight lines. We report a new detection of the 5797,Å DIB in the Red Rectangle nebula with an ETR, which adds a new observational constraint for the DIB carrier. We analysed archival GIRAFFE/FLAMES integral field spectroscopy to extract all spectral features between 5776 and 5810,Å at different spatial positions. We used time-series spectra to identify stellar and circumstellar features, and data from different instruments to exclude instrumental effects. We report the firm detection of the 5797,Å DIB that is localised in the Red Rectangle nebula, or in the ISM in the sight line towards it. Curiously, while the 5797,Å DIB shows an ETR in the Red Rectangle, the 5780,Å DIB does not. Further, we report an exceptionally high equivalent ratio of EW(5797)/EW(5780) = 1.79 This is higher than any other literature value.