Multiplanet system around bright (V=9) F-type star HD 106315.

A Multi-Planet System Transiting the V = 9 Rapidly Rotating F-Star HD 106315
https://arxiv.org/abs/1701.03807

Two Small Transiting Planets and a Third Body Orbiting HD 106315
https://arxiv.org/abs/1701.03811


Third planet is highly unconstrained.

Precise masses for the transiting planetary system HD 106315 with HARPS
https://arxiv.org/abs/1709.00865

Quote :

The multi-planetary system HD 106315 was recently found in K2 data . The planets have periods of Pb∼9.55 and Pc∼21.06days, and radii of rb=2.44±0.17 and rc=4.35±0.23 R⊕. The brightness of the host star (V=9.0 mag) makes it an excellent target for transmission spectroscopy. However, to interpret transmission spectra it is crucial to measure the planetary masses. We obtained high precision radial velocities for HD~106315 to determine the mass of the two transiting planets discovered with Kepler K2. Our successful observation strategy was carefully tailored to mitigate the effect of stellar variability. We modelled the new radial velocity data together with the K2 transit photometry and a new ground-based partial transit of HD 106315c to derive system parameters. We estimate the mass of HD 106315b to be 12.6 ± 3.2 M⊕ and the density to be 4.7±1.7gcm−3, while for HD 106315c we estimate a mass of 15.2 ± 3.7 M⊕ and a density of 1.01±0.29g\,cm−3. Hence, despite planet c having a radius almost twice as large as planet b, their masses are consistent with one another. We conclude that HD 106315c has a thick hydrogen-helium gaseous envelope. A detailed investigation of HD 106315b using a planetary interior model constrains the core mass fraction to be 5-29\%, and the water mass fraction to be 10-50\%. An alternative, not considered by our model, is that HD 106315b is composed of a large rocky core with a thick H-He envelope. Transmission spectroscopy of these planets will give insight into their atmospheric compositions and also help constrain their core compositions.

At the end of section 5, they note that they did not detect the RV drift reported by Crossfield et al that would have been indicative of a third planet.

The warm Neptunes around HD 106315 have low stellar obliquities
https://arxiv.org/abs/1807.00024

Quote :

We present the obliquity of the warm Neptune HD 106315c measured via a series of spectroscopic transit observations. HD 106315c is a 4.4 REarth warm Neptune orbiting a moderately rotating late F-star with a period of 21.05 days. HD 106315 also hosts a 2.5 REarth super-Earth on a 9.55 day orbit. Our Doppler tomographic analyses of four transits observed by the Magellan/MIKE, HARPS, and TRES facilities find HD 106315c to be in a low stellar obliquity orbit, consistent with being well aligned with the spin axis of the host star at lambda = -10 +3.6/-3.8 deg. We suggest, via dynamical N-body simulations, that the two planets in the system must be co-planar, and thus are both well aligned with the host star. HD 106315 is only the fourth warm Neptune system with obliquities measured. All warm Neptune systems have been found in well aligned geometries, consistent with the interpretation that these systems are formed in-situ in the inner protoplanetary disk, and also consistent with the majority of Kepler multi-planet systems that are in low obliquity orbits. With a transit depth of 1.02 mmag, HD 106315c is among the smallest planets to have been detected in transit spectroscopy, and we discuss its detection in the context of TESS and the next generations of spectrographs.