Number of UCXBs from the white dwarf donor channel (solid lines), helium-star donor channel (dashed lines) and main sequence donor channel (dotted lines). Time = 0 corresponds to the present. For each line style, the three lines represent the full population (upper) and the systems with orbital periods shorter than 60 min (middle) and shorter than 20 min (lower). A star formation history width σ = 0.5 Gyr has been used.