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Harvard ADS: Serendipitous observation of a white dwarf companion to a JWST/MIRI coronagraphic calibrator

Paper abstract: We present the unplanned detection of a white dwarf companion to the star HD 218261 in mid-infrared (10-16 \mum) observations with JWST/MIRI. This star was observed as a calibrator for coronagraphic observations of the exoplanet host HR 8799. HD 218261 B has only previously been detected by Gaia, and only in visible light. We confidently detect the companion in the mid-infrared, where it is less luminous than the primary by a factor of ~10^4. The visible and mid-infrared photometry are consistent with a white dwarf of T_\text{eff}~10000 K, M~0.8 M_\odot, though observation of its optical spectrum is required to precisely constrain its physical parameters. These results demonstrate that precise mid-infrared photometry of white dwarf companions to bright stars can be obtained with MIRI, opening up new possibilities for studying white dwarfs in close binaries.