Molly R. Kosiarek, University of California, Santa CruzFollow
David A. Berardo, Massachusetts Institute of TechnologyFollow
Ian J. M. Crossfield, University of KansasFollow
Cesar Laguna, University of California, Santa Cruz
Caroline Piaulet, Université de Montréal
Joseph M. Akana Murphy, University of California, Santa Cruz
Steve B. Howell, NASA Ames Research CenterFollow
Gregory W. Henry, Tennessee State UniversityFollow
Howard Isaacson, University of California - BerkeleyFollow
Benjamin Fulton, California Institute of TechnologyFollow
Lauren M. Weiss, University of HawaiiFollow
Erik A. Petigura, University of California, Los AngelesFollow
Aida Behmard, California Institute of TechnologyFollow
Lea A. Hirsch, Stanford UniversityFollow
Johanna Teske, Carnegie Institution of WashingtonFollow
Jennifer A. Burt, California Institute of TechnologyFollow
Sean M. Mills, California Institute of TechnologyFollow
Ashley Chontos, University of HawaiiFollow
Teo Močnik, NSF's NOIRLab
Andrew W. Howard, California Institute of TechnologyFollow
Michael Werner, California Institute of Technology
John H. Livingston, University of TokyoFollow
Jessica Krick, California Institute of Technology
Charles Beichman, California Institute of Technology
Varoujan Gorjian, California Institute of Technology
Laura Kreidberg, Max Planck Institute for AstronomyFollow
Caroline Morley, University of Texas at AustinFollow
Jessie L. Christiansen, California Institute of TechnologyFollow
Farisa Y. Morales, California Institute of TechnologyFollow
Nicholas J. Scott, NASA Ames Research CenterFollow
Jeffrey D. Crane, Observatories of the Carnegie Institution for ScienceFollow
Sharon Xuesong Wang, Observatories of the Carnegie Institution for ScienceFollow
Stephen A. Shectman, Observatories of the Carnegie Institution for ScienceFollow
Lee J. Rosenthal, California Institute of TechnologyFollow
Samuel K. Grunblatt, American Museum of Natural HistoryFollow
Ryan A. Rubenzahl, California Institute of TechnologyFollow
Paul A. Dalba, University of California, RiversideFollow
Steven Giacalone, University of California - Berkeley
Chiara Dane Villanueva, University of California, Santa Cruz
Qingtian Liu, University of California, Santa Cruz
Fei Dai, California Institute of TechnologyFollow
Michelle L. Hill, University of California, Riverside
Malena Rice, Yale University
Stephen R. Kane, University of California, RiversideFollow
Andrew W. Mayo, University of California - Berkeley

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HD 106315 and GJ 9827 are two bright, nearby stars that host multiple super-Earths and sub-Neptunes discovered by K2 that are well suited for atmospheric characterization. We refined the planets' ephemerides through Spitzer transits, enabling accurate transit prediction required for future atmospheric characterization through transmission spectroscopy. Through a multiyear high-cadence observing campaign with Keck/High Resolution Echelle Spectrometer and Magellan/Planet Finder Spectrograph, we improved the planets' mass measurements in anticipation of Hubble Space Telescope transmission spectroscopy. For GJ 9827, we modeled activity-induced radial velocity signals with a Gaussian process informed by the Calcium II H&K lines in order to more accurately model the effect of stellar noise on our data. We measured planet masses of Mb = 4.87 ± 0.37 M⊕, Mc = 1.92 ± 0.49 M⊕, and Md = 3.42 ± 0.62 M⊕. For HD 106315, we found that such activity radial velocity decorrelation was not effective due to the reduced presence of spots and speculate that this may extend to other hot stars as well (Teff > 6200 K). We measured planet masses of Mb = 10.5 ± 3.1 M⊕ and Mc = 12.0 ± 3.8 M⊕. We investigated all of the planets' compositions through comparison of their masses and radii to a range of interior models. GJ 9827 b and GJ 9827 c are both consistent with a 50/50 rock-iron composition, GJ 9827 d and HD 106315 b both require additional volatiles and are consistent with moderate amounts of water or hydrogen/helium, and HD 106315 c is consistent with a ∼10% hydrogen/helium envelope surrounding an Earth-like rock and iron core.