Unresolved binary systems with white dwarfs in open star clusters

We invesigate unresolved binary systems with components of main sequence star (MS) and white dwarf (WD) in nine open clusters. These systems are located below and to the left of the main sequence at the colour-magnitude diagram. We compare the number of cluster stars, which have likely evolved into white dwarfs, with the number of candidates for unresolved binary systems with WD. The number of probable cluster members, lying below the main sequence, is generally less than the expected number of white dwarfs. The observations in the ultraviolet could detect WDs and unresolved binary WD+MS systems more confidently than the observations in the visible range.