on distances up to 300 pc and more. Below we will consider the star clusters in the range of separation where the linear correlation between colour and separation is observed.

After preliminary selection, the sample included 741 pair candidates in NGC 628, 116 pairs in NGC 3184, 297 pairs in NGC 3726, 126 pairs in NGC 5585, and 808 pairs in NGC 6946.

A native source extractor selection is known to include a significant number of non-cluster contaminants after generating the star clusters data base [@grasha2015]. A good test for estimation of the contamination of our star cluster catalogue and its clearing from non-cluster objects, such as background stars and galaxies, and cosmic rays, are $U-B$ colours [@grasha2019]. We checked the disposition of our star cluster candidates on the $(U-B)-(B-V)$ colour-colour diagram (Fig. \[figure:ub\_logs\]).

We left for further consideration star cluster candidates whose colours, within maximum allowable errors, are consistent with the colours of SSP CMD evolutionary tracks for stellar systems younger than 1 Gyr (black dots in Fig. \[figure:ub\_logs\]). The red limit in $(U-B)^i_0=0.54$ mag is also the boundary which separates young star clusters with $B-V\sim0.5$ mag from the background Main Sequence yellow stars [@whitmore2010].

A direct unambiguous relation 