  We report the discovery of HATS-15 b and HATS-16 b, two massive transiting
extrasolar planets orbiting evolved ($\sim 10$ Gyr) main-sequence stars. The
planet HATS-15 b, which is hosted by a G9V star ($V=14.8$ mag), is a hot
Jupiter with mass of $2.17\pm0.15\, M_{\mathrm{J}}$ and radius of
$1.105\pm0.0.040\, R_{\mathrm{J}}$, and completes its orbit in nearly 1.7 days.
HATS-16 b is a very massive hot Jupiter with mass of $3.27\pm0.19\,
M_{\mathrm{J}}$ and radius of $1.30\pm0.15\, R_{\mathrm{J}}$; it orbits around
its G3 V parent star ($V=13.8$ mag) in $\sim2.7$ days. HATS-16 is slightly
active and shows a periodic photometric modulation, implying a rotational
period of 12 days which is unexpectedly short given its isochronal age. This
fast rotation might be the result of the tidal interaction between the star and
its planet.
