Two Warm Neptunes transiting HIP 9618 revealed by TESS &amp; Cheops
Creators: name: Osborn, Hugh P.
name: Nowak, Grzegorz
name: Hébrard, Guillaume
name: Masseron, Thomas
name: Lillo-Box, J.
name: Pallé, Enric
name: Bekkelien, Anja
name: Florén, Hans-Gustav
name: Guterman, Pascal
name: Simon, Attila E.
name: Adibekyan, V.
name: Bieryla, Allyson
name: Borsato, Luca
name: Brandeker, Alexis
name: Ciardi, David R.
name: Cameron, Andrew Collier
name: Collins, Karen A.
name: Egger, Jo A.
name: Gandolfi, Davide
name: Hooton, Matthew J.
name: Latham, David W.
name: Lendl, Monika
name: Matthews, Elisabeth C.
name: Tuson, Amy
name: Ulmer-Moll, Solène
name: Vanderburg, Andrew
name: Wilson, Thomas G.
name: Ziegler, Carl
name: Alibert, Yann
name: Alonso, Roi
name: Anglada, Guillem
name: Arnold, Luc
name: Asquier, Joel
name: Navascues, David Barrado y
name: Baumjohann, Wolfgang
name: Beck, Thomas
name: Belinski, Alexandr A.
name: Benz, Willy
name: Biondi, Federico
name: Boisse, Isabelle
name: Bonfils, Xavier
name: Broeg, Christopher
name: Buchhave, Lars A.
name: Bárczy, Tamas
name: Barros, S. C. C.
name: Cabrera, Juan
name: Guillen, Carlos Cardona
name: Carleo, Ilaria
name: Castro-González, Amadeo
name: Charnoz, Sébastien
name: Christiansen, Jessie
name: Cortes-Zuleta, Pia
name: Csizmadia, Szilard
name: Dalal, Shweta
name: Davies, Melvyn B.
name: Deleuil, Magali
name: Delfosse, Xavier
name: Delrez, Laetitia
name: Demory, Brice-Olivier
name: Dunlavey, Ava B.
name: Ehrenreich, David
name: Erikson, Anders
name: Fernandes, Rachel B.
name: Fortier, Andrea
name: Forveille, Thierry
name: Fossati, Luca
name: Fridlund, Malcolm
name: Gillon, Michaël
name: Goeke, Robert F.
name: Goliguzova, Maria V.
name: Gonzales, Erica J.
name: Günther, M. N.
name: Güdel, Manuel
name: Heidari, Neda
name: Henze, Christopher E.
name: Howell, Steve
name: Hoyer, Sergio
name: Frey, Jonas Immanuel
name: Isaak, Kate G.
name: Jenkins, Jon M.
name: Kiefer, Flavien
name: Kiss, Laszlo
name: Korth, Judith
name: Maxted, Pierre F. L.
name: Laskar, Jacques
name: Etangs, Alain Lecavelier des
name: Lovis, Christophe
name: Lund, Michael B.
name: Luque, Rafa
name: Magrin, Demetrio
name: Almenara, Jose Manuel
name: Martioli, Eder
name: Mecina, Marko
name: Medina, Jennifer V.
name: Moldovan, Daniel
name: Morales-Calderón, María
name: Morello, Giuseppe
name: Moutou, Claire
name: Murgas, Felipe
name: Jensen, Eric L. N.
name: Nascimbeni, Valerio
name: Olofsson, Göran
name: Ottensamer, Roland
name: Pagano, Isabella
name: Peter, Gisbert
name: Piotto, Giampaolo
name: Pollacco, Don
name: Queloz, Didier
name: Ragazzoni, Roberto
name: Rando, Nicola
name: Rauer, Heike
name: Ribas, Ignasi
name: Ricker, George
name: Demangeon, Olivier D. S.
name: Smith, Alexis M. S.
name: Santos, Nuno
name: Scandariato, Gaetano
name: Seager, Sara
name: Sousa, Sergio G.
name: Steller, Manfred
name: Szabó, Gyula M.
name: Ségransan, Damien
name: Thomas, Nicolas
name: Udry, Stéphane
name: Ulmer, Bernd
name: Van Grootel, Valerie
name: Vanderspek, Roland
name: Walton, Nicholas
name: Winn, Joshua N.
Publisher: name: arXiv
Year: 2023
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 $3.9 \pm 0.044$ $R_\oplus$ (HIP 9618 b) and $3.343 \pm 0.039$ $R_\oplus$ (HIP 9618 c). While the 20.77291 day period of HIP 9618 b was measured unambiguously, HIP 9618 c showed only two transits separated by a 680-day 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 $10.0 \pm 3.1 M_\oplus$ for HIP 9618 b, which, according to our interior structure models, corresponds to a $6.8\pm1.4\%$ gas fraction. HIP 9618 c appears to have a lower mass than HIP 9618 b, with a 3-sigma upper limit of $&lt; 18M_\oplus$. 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 ($0.08^{+0.12}_{-0.05} M_\odot$) orbiting at $26^{+19}_{-11}$ au. This detection makes HIP 9618 one of only five bright ($K&lt;8$ mag) transiting multi-planet systems known to host a planet with $P&gt;50$ d, opening the door for the atmospheric characterisation of warm ($T_{\rm eq}&lt;750$ K) sub-Neptunes.
description: 19 pages, 16 figures, 9 tables. Accepted at MNRAS. CHEOPS, RV and ground-based photometric data is available on CDS at https://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/523/3069