Context : 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 .

Aims : By using new photometric data , we computed precise estimates of the physical properties of two transiting planetary systems at equilibrium temperature of \sim 2000 K .

Methods : We present new broad-band , multi-colour , photometric observations obtained using three small class telescopes and the telescope-defocussing technique .
In particular we obtained 11 and 10 light curves covering 8 and 7 transits of HAT-P-23 and WASP-48 respectively .
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 jktebop and from published spectroscopic measurements .

Results : 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 _ { \mathrm { b } } = 1.224 \pm 0.037 R _ { \mathrm { Jup } } and R _ { \mathrm { b } } = 1.396 \pm 0.051 R _ { \mathrm { Jup } } , respectively , than those measured in the discovery papers ( R _ { \mathrm { b } } = 1.368 \pm 0.090 R _ { \mathrm { Jup } } and R _ { \mathrm { b } } = 1.67 \pm 0.10 R _ { \mathrm { Jup } } ) .
The density of the two planets are higher than those previously published ( \rho _ { \mathrm { b } } \sim 1.1 and \sim 0.3 \rho _ { \mathrm { 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 .

Conclusions :