  We report the spectroscopic confirmation of the Kepler object of interest
KOI-183.01 (Kepler-423b), a half-Jupiter mass planet transiting an old
solar-like star every 2.7 days. Our analysis is the first to combine the full
Kepler photometry (quarters 1-17) with high-precision radial velocity
measurements taken with the FIES spectrograph at the Nordic Optical Telescope.
We simultaneously modelled the photometric and spectroscopic data-sets using
Bayesian approach coupled with Markov chain Monte Carlo sampling. We found that
the Kepler pre-search data conditioned (PDC) light curve of KOI-183 exhibits
quarter-to-quarter systematic variations of the transit depth, with a
peak-to-peak amplitude of about 4.3 % and seasonal trends reoccurring every
four quarters. We attributed these systematics to an incorrect assessment of
the quarterly variation of the crowding metric. The host star KOI-183 is a G4
dwarf with $M_\star=0.85\pm0.04$ M$_\rm{Sun}$, $R_\star=0.95\pm0.04$
R$_\rm{Sun}$, $T_\mathrm{eff}=5560\pm80$ K, $[M/H]=-0.10\pm0.05$ dex, and with
an age of $11\pm2$ Gyr. The planet KOI-183b has a mass of
$M_\mathrm{p}=0.595\pm0.081$ M$_\mathrm{Jup}$ and a radius of
$R_\mathrm{p}=1.192\pm0.052$ R$_\mathrm{Jup}$, yielding a planetary bulk
density of $\rho_\mathrm{p}=0.459\pm0.083$ g/cm$^{3}$. The radius of KOI-183b
is consistent with both theoretical models for irradiated coreless giant
planets and expectations based on empirical laws. The inclination of the
stellar spin axis suggests that the system is aligned along the line of sight.
We detected a tentative secondary eclipse of the planet at a 2-$\sigma$
confidence level ($\Delta F_{\mathrm{ec}}=14.2\pm6.6$ ppm) and found that the
orbit might have a small non-zero eccentricity of $e=0.019^{+0.028}_{-0.014}$.
With a Bond albedo of $A_\mathrm{B}=0.037\pm0.019$, KOI-183b is one of the
gas-giant planets with the lowest albedo known so far.
