TOI-1259Ab – a gas giant planet with 2.7 per cent deep transits and a bound white dwarf companion

We present TOI-1259Ab, a 1.0 Rjup gas giant planet transiting 0.71 Rsun K-dwarf on 3.48 day orbit. The system also contains bound white dwarf companion TOI-1259B with projected distance of approximately 1600 AU from the host. Transits are observed in nine TESS sector and 2.7 per cent deep - among deepest known making TOI-1259Ab promising target for atmospheric characterization. Our follow-up radial velocity measurements indicate variability semiamplitude K = 71 m/s, implying mass 0.44 Mjup. By fitting spectral energy distribution we derive total age 4.08 (+1.21 -0.53) Gyr system. K-dwarf's light curve reveals rotational period 28 days, which implies gyrochronology broadly consistent dwarf's age.