Two Warm Neptunes Transiting HIP 9618 Revealed by TESS and Cheops

HIP 9618 (HD 12572, TOI-1471, TIC 306263608) is a bright (G = 9.0 mag) solar analogue. TESS photometry revealed the star to have two candidate planets with radii of 3.9 +/- 0.044 R-circle plus (HIP 9618 b) and 3.343 +/- 0.039 R-circle plus (HIP 9618 c). While the 20.77291 d period of HIP 9618 b was measured unambiguously, HIP 9618 c showed only two transits separated by a 680-d gap in the time series, leaving many possibilities for the period. To solve this issue, CHEOPS performed targeted photometry of period aliases to attempt to recover the true period of planet c, and successfully determined the true period to be 52.56349 d. High-resolution spectroscopy with HARPS-N, SOPHIE, and CAFE revealed a mass of 10.0 +/- 3.1M(circle plus) for HIP 9618 b, which, according to our interior structure models, corresponds to a 6.8 +/- 1.4 per cent gas fraction. HIP 9618 c appears to have a lower mass than HIP 9618 b, with a 3-sigma upper limit of <18M(circle plus). Follow-up and archival RV measurements also reveal a clear long-term trend which, when combined with imaging and astrometric information, reveal a low-mass companion (0.08(-0.05)(+0.12) M-circle dot) orbiting at 26.0(-11.0)(+19.0) au. This detection makes HIP 9618 one of only five bright (K < 8 mag) transiting multiplanet systems known to host a planet with P > 50 d, opening the door for the atmospheric characterization of warm (T-eq < 750 K) sub-Neptunes.