HIP 9618 light and radial velocity curves

TESS photometry revealed the star to have two candidate planets with radii of 3.9+/-0.044 R_{Earth}_ (HIP 9618 b) and 3.343+/-0.039R_{Earth}_ (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_{Earth}_ 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_{Earth}_.