The interstellar extinction law in twenty open star clusters namely Berkeley 7, Collinder 69, Hogg 10, NGC 2362, Czernik 43, NGC 6530, NGC 6871, Bochum 10, Haffner 18, IC 4996, NGC 2384, NGC 6193, NGC 6618, NGC 7160, Collinder 232, Haffner 19, NGC 2401, NGC 6231, NGC 6823 and NGC 7380 have been studied in the optical and near-IR wavelength ranges. The difference between maximum and minimum values of E(B-V) indicates the presence of non-uniform extinction in all the clusters except Collinder 69, NGC 2362 and NGC 2384. The colour excess ratios are consistent with a normal extinction law for the clusters NGC 6823, Haffner 18, Haffner 19, NGC 7160, NGC 6193, NGC 2401, NGC 2384, NGC 6871, NGC 7380, Berkeley 7, Collinder 69 and IC 4996. We found that differential colour-excess which may be due to the occurrence of dust and gas inside the clusters, decreases with age of the clusters. A spatial variation of colour excess is found in NGC 6193 in sense that it decreases from east to west in cluster region. For cluster Berkeley 7, NGC 7380 and NGC 6871, a dependence of colour excess with spectral class and luminosity is observed. Eight stars in Collinder 232, four stars in NGC 6530 and one star in NGC 6231 have colour excess flux in near-IR. This indicates that these stars may have circumstellar material around them.
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