  We present and confirm TOI-1751 b, a transiting sub-Neptune orbiting a
slightly evolved, solar-type, metal-poor star ($T_{eff} = 5996 \pm 110$ K,
$log(g) = 4.2 \pm 0.1$, V = 9.3 mag, [Fe/H] = $-0.40 \pm 0.06$ dex) every 37.47
d. We use TESS photometry to measure a planet radius of
$2.77_{-0.07}^{+0.15}~\rm{R_\oplus}$. We also use both Keck/HIRES and APF/Levy
radial velocities (RV) to derive a planet mass of $14.5_{-3.14}^{+3.15}
~\rm{M_\oplus}$, and thus a planet density of $3.6 \pm 0.9 \, {\rm g}\,{\rm
cm}^{-3}$. There is also a long-period ($\sim400~\rm{d}$) signal that is
observed in only the Keck/HIRES data. We conclude that this long-period signal
is not planetary in nature, and is likely due to the window function of the
Keck/HIRES observations. This highlights the role of complementary observations
from multiple observatories to identify and exclude aliases in RV data.
Finally, we investigate potential compositions of this planet, including rocky
and water-rich solutions, as well as theoretical irradiated ocean models.
TOI-1751 b is a warm sub-Neptune, with an equilibrium temperature of $\sim 820$
K. As TOI-1751 is a metal-poor star, TOI-1751 b may have formed in a
water-enriched formation environment. We thus favor a volatile-rich interior
composition for this planet.
