TOI-431/HIP 26013: a super-Earth and a sub-Neptune transiting a bright, early K dwarf, with a third RV planet
Authors: Osborn Ares , Armstrong David J , Cale Bryson , Brahm Rafael , Wittenmyer Robert A , Dai Fei , Crossfield Ian J M , Bryant Edward M , Adibekyan Vardan , Cloutier Ryan , Collins Karen A , Delgado Mena E , Fridlund Malcolm , Hellier Coel , Howell Steve B , King George W , Lillo-Box Jorge , Otegi Jon , Sousa S , Stassun Keivan G , Matthews Elisabeth C , Ziegler Carl , Ricker George , Vanderspek Roland , Latham David W , Seager S , Winn Joshua N , Jenkins Jon M , Acton Jack S , Addison Brett C , Anderson David R , Ballard Sarah , Barrado David , Barros Susana C C , Batalha Natalie , Bayliss Daniel , Barclay Thomas , Benneke Björn , Berberian John , Bouchy Francois , Bowler Brendan P , Briceño César , Burke Christopher J , Burleigh Matthew R , Casewell Sarah L , Ciardi David , Collins Kevin I , Cooke Benjamin F , Demangeon Olivier D S , Díaz Rodrigo F , Dorn C , Dragomir Diana , Dressing Courtney , Dumusque Xavier , Espinoza Néstor , Figueira P , Fulton Benjamin , Furlan E , Gaidos E , Geneser C , Gill Samuel , Goad Michael R , Gonzales Erica J , Gorjian Varoujan , Günther Maximilian N , Helled Ravit , Henderson Beth A , Henning Thomas , Hogan Aleisha , Hojjatpanah Saeed , Horner Jonathan , Howard Andrew W , Hoyer Sergio , Huber Dan , Isaacson Howard , Jenkins James S , Jensen Eric L N , Jordán Andrés , Kane Stephen R , Kidwell Richard C , Kielkopf John , Law Nicholas , Lendl Monika , Lund M , Matson Rachel A , Mann Andrew W , McCormac James , Mengel Matthew W , Morales Farisa Y , Nielsen Louise D , Okumura Jack , Osborn Hugh P , Petigura Erik A , Plavchan Peter , Pollacco Don , Quintana Elisa V , Raynard Liam , Robertson Paul , Rose Mark E , Roy Arpita , Reefe Michael , Santerne Alexandre , Santos Nuno C , Sarkis Paula , Schlieder J , Schwarz Richard P , Scott Nicholas J , Shporer Avi , Smith A M S , Stibbard C , Stockdale Chris , Strøm Paul A , Twicken Joseph D , Tan Thiam-Guan , Tanner A , Teske J , Tilbrook Rosanna H , Tinney C G , Udry Stephane , Villaseñor Jesus Noel , Vines Jose I , Wang Sharon X , Weiss Lauren M , West Richard G , Wheatley Peter J , Wright Duncan J , Zhang Hui , Zohrabi F 
Published: August 2021

We present the bright (Vmag = 9.12), multiplanet system TOI-431, characterized with photometry and radial velocities (RVs). We estimate the stellar rotation period to be 30.5 ± 0.7 d using archival photometry and RVs. Transiting Exoplanet Survey Satellite (TESS) objects of Interest (TOI)-431 b is a super-Earth with a period of 0.49 d, a radius of 1.28 ± 0.04 R⊕, a mass of 3.07 ± 0.35 M⊕, and a density of 8.0 ± 1.0 g cm−3; TOI-431 d is a sub-Neptune with a period of 12.46 d, a radius of 3.29 ± 0.09 R⊕, a mass of $9.90^{+1.53}_{-1.49}$ M⊕, and a density of 1.36 ± 0.25 g cm−3. We find a third planet, TOI-431 c, in the High Accuracy Radial velocity Planet Searcher RV data, but it is not seen to transit in the TESS light curves. It has an Msin i of $2.83^{+0.41}_{-0.34}$ M⊕, and a period of 4.85 d. TOI-431 d likely has an extended atmosphere and is one of the most well-suited TESS discoveries for atmospheric characterization, while the super-Earth TOI-431 b may be a stripped core. These planets straddle the radius gap, presenting an interesting case-study for atmospheric evolution, and TOI-431 b is a prime TESS discovery for the study of rocky planet phase curves.