  Accurate and repeated photometric follow-up observations of planetary-transit
events are important to precisely characterize the physical properties of
exoplanets. A good knowledge of the main characteristics of the exoplanets is
fundamental to trace their origin and evolution. Multi-band photometric
observations play an important role in this process. By using new photometric
data, we computed precise estimates of the physical properties of two
transiting planetary systems. We present new broad-band, multi-colour,
photometric observations obtained using three small class telescopes and the
telescope-defocussing technique. For each of the two targets, one transit event
was simultaneously observed through four optical filters. One transit of
WASP-48 b was monitored with two telescopes from the same observatory. The
physical parameters of the systems were obtained by fitting the transit light
curves with {\sc jktebop} and from published spectroscopic measurements. We
have revised the physical parameters of the two planetary systems, finding a
smaller radius for both HAT-P-23 b and WASP-48 b, $R_{b}=1.224 \pm 0.037
R_{Jup}$ and $R_{b}=1.396 \pm 0.051 \, R_{Jup}$, respectively, than those
measured in the discovery papers ($R_{b}=1.368 \pm 0.090 R_{Jup}$ and
$R_{b}=1.67 \pm 0.10 R_{Jup}$). The density of the two planets are higher than
those previously published ($\rho_{b}$ ~1.1 and ~0.3 $\rho_{jup}$ for HAT-P-23
and WASP-48 respectively) hence the two Hot Jupiters are no longer located in a
parameter space region of highly inflated planets. An analysis of the variation
of the planet's measured radius as a function of optical wavelength reveals
flat transmission spectra within the experimental uncertainties. We also
confirm the presence of the eclipsing contact binary NSVS-3071474 in the same
field of view of WASP-48, for which we refine the value of the period to be
0.459 d.
