Orbital period distribution. Top panel: Histogram of the orbital period of UCXBs and short-period LMXB systems, plotted in 10 min bins (grey) and 5 min bins [blue for all systems (field + GC) and green for those located in GCs]. The canonical 80 min that defines the bona fide UCXBs limit is shown as a dotted black line. Lower panel: Percentage of systems accreting persistently.