in order to describe in detail the advantages of using the combination of $F160BW$ and $F555W$ magnitudes for comparison with HB models.\
\
In all cases the photometric analysis has been performed by using ROMAFOT (Buonanno et al. 1983). The reduction strategy has been optimized to be particularly sensitive to hot stars, as already described by Ferraro et al. (2003). For the present analysis attention has been paid to the cross-identification of stars detected in the $F160BW$ images. Stars observed only in these bands have been force-fitted in the other bands (see Paper I).\
The instrumental magnitudes have been calibrated to the VEGAMAG photometric system by using zero-points and the gain settings reported in Table 5.1 of the WFPC2 *data handbook Manual*[^2]. Magnitudes thus obtained have been also corrected for Charge Transfer Efficiency (CTE) effect by means of the prescriptions by Dolphin (2000) and updated equations listed in the dedicated page of Dolphin’s web site[^3].\
The resulting colour Magnitude Diagrams (CMDs) for each cluster are shown in both the ($m_{F160BW}$, $m_{F160BW} -m_{F555W}$) and ($m_{F336W}$, $m_{F336W}-m_{F555W}$) planes in Figs \[cmdopt\] and  \[cmdfuv\].

![Far-UV CMDs of M13 and M79.[]{data-label="cmdfuv"}](f2.ps){width="84mm"}

Star lists have been reported to the absolute coordinate system by mean of 