Low-resolution LT/SPRAT spectrum (panel a), and the NTT/SOFI (2021 May 9) and the IRTF/SpeX (2023 December 3) low- and medium-resolution flux-calibrated spectra of Gaia20bdk (panel b). The grey-shaded areas in this panel demarcate regions strongly affected by telluric lines. The IRTF spectrum and photometry were shifted up by 3 × 10⁻¹⁵ erg cm⁻¹ s⁻¹ Å⁻¹ for clarity. Only the major spectral lines and bands characteristic of FUors are indicated. Telluric absorption bands were not removed from the visual spectrum.