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Galactic extinction laws: II. Hidden in plain sight, a new interstellar absorption band at 7700 Å broader than any known DIB

We have detected a broad interstellar absorption band centred close to 7700 Å and with a FWHM of 176.6$\pm$3.9 Å. This is the first such absorption band detected in the optical range and is significantly wider than the numerous diffuse interstellar bands (DIBs). It remained undiscovered until now because it is partially hidden behind the A telluric band produced by O$_2$. The band was discovered using STIS@HST spectra and later detected in a large sample of stars of diverse type (OB stars, BA supergiants, red giants) using further STIS and ground-based spectroscopy. The EW of the band is measured and compared with our extinction and K I $λλ$7667.021,7701.093 measurements for the same sample. The carrier is ubiquitous in the diffuse and translucent Galactic ISM but is depleted in the environment around OB stars. In particular, it appears to be absent or nearly so in sightlines rich in molecular carbon. This behaviour is similar to that of the $σ$-type DIBs, which originate in the low/intermediate-density UV-exposed ISM but are depleted in the high-density UV-shielded molecular clouds. We also present an update on our previous work on the relationship between $E(4405-5495)$ and $R_{5495}$ and incorporate our results into a general model of the ISM.