  We present an atmospheric characterization study of two medium sized planets
bracketing the radius of Neptune: HD 106315 c (R$_{\rm{P}}$=4.98 $\pm$ 0.23
R$_{\oplus}$) and HD 3167 c (R$_{\rm{P}}$=2.740$_{-0.100}^{+0.106}$
R$_{\oplus}$). We analyse spatially scanned spectroscopic observations obtained
with the G141 grism (1.125 - 1.650 $\mu$m) of the Wide Field Camera 3 (WFC3)
onboard the Hubble Space Telescope. We use the publicly available Iraclis
pipeline and TauREx3 atmospheric retrieval code and we detect water vapor in
the atmosphere of both planets with an abundance of
$\log_{10}[\mathrm{H_2O}]=-2.1^{+0.7}_{-1.3}$ ($\sim$5.68$\sigma$) and
$\log_{10}[\mathrm{H_2O}]=-4.1^{+0.9}_{-0.9}$ ($\sim$3.17$\sigma$) for HD
106315 c and HD 3167 c, respectively. The transmission spectrum of HD 106315 c
shows also a possible evidence of ammonia absorption ($\log_{10}[\mathrm
{NH_3}]=-4.3^{+0.7}_{-2.0}$, $\sim$1.97$\sigma$ -even if it is not
significant-), whilst carbon dioxide absorption features may be present in the
atmosphere of HD 3167 c in the $\sim$1.1-1.6~$\mu$m wavelength range
($\log_{10}[\mathrm{CO_{2}}]= -2.4^{+0.7}_{-1.0}$, $\sim$3.28$\sigma$). However
the CO$_2$ detection appears significant, it must be considered carefully and
put into perspective. Indeed, CO$_2$ presence is not explained by 1D
equilibrium chemistry models, and it could be due to possible systematics. The
additional contribution of clouds, CO and CH$_4$ are discussed. HD 106315 c and
HD 3167 c will be interesting targets for upcoming telescopes such as the James
Webb Space Telescope (JWST) and the Atmospheric Remote-Sensing Infrared
Exoplanet Large-Survey (Ariel).
