within a wide range of $S$ for all five studied galaxies.

Note here that typical $U-B$ colours differ slightly from galaxy to galaxy (Fig. \[figure:col\_dist\]). This may be a result of zero-point calibration errors and/or uncertainty of Galactic or internal extinctions. A special case is NGC 6946 which is located on the low Galactic latitude (see column (10) in Table \[table:sample\]).

Unlike @efremov1998 and @grasha2017, where authors calculated the age difference in star cluster pairs, we can use colour index $U-B$, corrected for Galactic extinction and inclination effects, as a direct age indicator. The age of H[ii]{} bubbles (H$\alpha$ sources) is typically equal to a few Myr [@whitmore2011]. The age of star clusters without H$\alpha$ emission is tens and hundreds Myr (see Figs. \[figure:ubv\_sep\] and \[figure:ubv\_age\]). Thus $\Delta t \equiv 
t({\rm SC})-t({\rm H}{\textsc {ii}}{\rm R})\approx t({\rm SC})$.

As we noted above, star clusters without H$\alpha$ emission are free from dust [@whitmore2011]. Thus, we can estimate age of clusters by their colour indices, corrected for Galactic extinction and inclination effects, using evolutionary models. Variations of $U-B$ colour index for ageing synthetic stellar systems are shown in Fig. \[figure:ubt\].

  ---------- ---------------- --------------- --------------- --------------- --------------- --------------- --------------- --------------- ---------------
    Galaxy                                                                                                                                    
                                                                                                                                              
                 $\alpha$         $\beta$      