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HD 106315 c
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Title: Ground-based photometry of the 21-day Neptune HD106315c
Author: M. Lendl (1,2), D. Ehrenreich (2), O.D.Turner (2), D. Bayliss (2), S. Blanco-Cuaresma (2,3), H. Giles (2), F. Bouchy (2), M. Marmier (2), S. Udry (2) ((1) Space Research Institute, Austrian Academy of Sciences (2) Observatoire de Genève, Université de Genève, (3) Harvard-Smithsonian Center for Astrophysics)

Space-based transit surveys such as K2 and TESS allow the detection of small transiting planets with orbital periods beyond 10 days. Few of these warm Neptunes are currently known around stars bright enough to allow for detailed follow-up observations dedicated to their atmospheric characterization. The 21-day period and 3.95 earth radii planet HD106315c has been discovered based on the observation of two of its transits by K2. We have observed HD106315 using the 1.2m Euler telescope equipped with the EulerCam camera on two instances to confirm the transit using broad band photometry and refine the planetary period. Based on two observed transits of HD106315c, we detect its ~1 mmag transit and obtain a precise measurement of the planetary ephemerids, which are critical for planning further follow-up observations. We have used the attained precision together with the predicted yield from the TESS mission to evaluate the potential for ground-based confirmation of Neptune-sized planets found by TESS. We find that 1-meter-class telescopes on the ground equipped with precise photometers could substantially contribute to the follow-up of 162 TESS candidates orbiting stars with magnitudes of V \le q14. Out of these, 74 planets orbit stars with V \leq 12 and 12 planets orbit V \leq 10, which makes these candidates high-priority objects for atmospheric characterization with high-end instrumentation.

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HD 106315
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Title: Two Small Transiting Planets and a Third Body Orbiting HD 106315
Author: Ian J. M. Crossfield, David R. Ciardi, Howard Isaacson, Andrew W. Howard, Erik A. Petigura, Lauren M. Weiss, Benjamin J. Fulton, Evan Sinukoff, Joshua E. Schlieder, Dimitri Mawet, Garreth Ruane, Imke de Pater, Katherine de Kleer, Ashley G. Davies, Jessie L. Christiansen, Courtney D. Dressing, Lea Hirsch, Björn Benneke, Justin R. Crepp, Molly Kosiarek, John Livingston, Erica Gonzales, Charles A. Beichman, Heather A. Knutson

The masses, atmospheric makeups, spin-orbit alignments, and system architectures of extrasolar planets can be best studied when the planets orbit bright stars. We report the discovery of three bodies orbiting HD 106315, a bright (V = 8.97 mag) F5 dwarf targeted by our K2 survey for transiting exoplanets. Two small, transiting planets have radii of 2.23 (+0.30/-0.25) R_Earth and 3.95 (+0.42/-0.39) R_Earth and orbital periods of 9.55 d and 21.06 d, respectively. A radial velocity (RV) trend of 3.55 ± 0.10 m/s/day indicates the presence of a third body orbiting HD 106315 with period >80 d and mass >M_Jup. Transits of this object would have depths of ~1% and are definitively ruled out. Though the star has v sin i=13.2 km/s, it exhibits short-timescale RV variability of just 6.4 m/s, and so is a good target for RV measurements of the mass and density of the inner two planets and the outer object's orbit and mass. Furthermore, the combination of RV jitter and moderate v sin i makes HD 106315 a valuable laboratory for studying the spin-orbit alignment of small planets through the Rossiter-McLaughlin effect. Space-based atmospheric characterization of the two transiting planets via transit and eclipse spectroscopy should also be feasible. This discovery demonstrates again the power of K2 to find compelling exoplanets worthy of future study.

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