Cumulative probability distribution of UCXB bolometric luminosity based on ASM data (fraction of time that a source emits above a given luminosity) for 14 UCXBs including two candidates with a tentative orbital period. Only count rates that are almost certainly real (their significant detection is almost certainly not the result of noise) have been used (Sects. 2.3.1 and 3.1). The labels indicate the orbital period in min of the corresponding systems. Different line styles are used only to distinguish different systems.