Spectroscopy of TOI-1259B-an unpolluted white dwarf companion to an inflated warm Saturn

TOI-1259 consists of a transiting exoplanet orbiting a main-sequence star, with a bound outer white dwarf (WDs) companion. Less than a dozen systems with this architecture are known. We conduct follow-up spectroscopy on the WD TOI-1259B using the Large Binocular Telescope to better characterize it. We observe only strong hydrogen lines, making TOI-1259B a DA WD. We see no evidence of heavy element pollution, which would have been evidence of planetary material around the WD. Such pollution is seen in similar to 25-50 per cent of WDs, but it is unknown if this rate is higher or lower in TOI-1259-like systems that contain a known planet. Our spectroscopy permits an improved WD age measurement of 4.05(-0.42)(+1.00) Gyr, which matches gyrochronology of the main-sequence star. This is the first of an expanded sample of similar binaries that will allow us to calibrate these dating methods and provide a new perspective on planets in binaries.

Automated summary

TOI-1259 is a transiting exoplanet with a bound outer white dwarf companion. Spectroscopy confirms that the white dwarf, TOI-1259B, is a DA type without evidence of heavy element pollution. The improved WD age measurement matches the gyrochronology of the main-sequence star and offers new perspectives on planets in binaries.