  Radial velocity (RV) measurements of transiting multiplanet systems allow us
to understand the densities and compositions of planets unlike those in the
Solar System. Kepler-102, which consists of 5 tightly packed transiting
planets, is a particularly interesting system since it includes a super-Earth
(Kepler-102d) and a sub-Neptune-sized planet (Kepler-102e) for which masses can
be measured using radial velocities. Previous work found a high density for
Kepler-102d, suggesting a composition similar to that of Mercury, while
Kepler-102e was found to have a density typical of sub-Neptune size planets;
however, Kepler-102 is an active star, which can interfere with RV mass
measurements. To better measure the mass of these two planets, we obtained 111
new RVs using Keck/HIRES and TNG/HARPS-N and modeled Kepler-102's activity
using quasi-periodic Gaussian Process Regression. For Kepler-102d, we report a
mass upper limit of M$_{d} < $5.3 M$_{\oplus}$ [95\% confidence], a best-fit
mass of M$_{d}$=2.5 $\pm$ 1.4 M$_{\oplus}$, and a density of $\rho_{d}$=5.6
$\pm$ 3.2 g/cm$^{3}$ which is consistent with a rocky composition similar in
density to the Earth. For Kepler-102e we report a mass of M$_{e}$=4.7 $\pm$ 1.7
M$_{\oplus}$ and a density of $\rho_{e}$=1.8 $\pm$ 0.7 g/cm$^{3}$. These
measurements suggest that Kepler-102e has a rocky core with a thick gaseous
envelope comprising 2-4% of the planet mass and 16-50% of its radius. Our study
is yet another demonstration that accounting for stellar activity in stars with
clear rotation signals can yield more accurate planet masses, enabling a more
realistic interpretation of planet interiors.
