  The period distribution of contact binaries exhibits a very sharp short
period cut-off at 0.22 days. In order to provide valuable information on this
short period limit, we observed ten totally eclipsing contact binaries with
orbital periods near this cut-off. By detailed analysis using W-D code, we
determined that two of these systems are A-subtype contact binaries while the
others are W-subtype contact binaries and all the targets show shallow contact
configurations. Half of the targets exhibit stellar spot activity and four of
them have third light. A statistical work on well studied USPCBs was carried
out, and physical parameters of fifty-five USPCBs were obtained. Some common
properties for these systems were derived. Due to the study of the period-color
diagram of USPCBs, we found that the period-color relation of USPCBs is
different from other W UMa type contact binaries and USPCBs are metal poor old
stars. In addition, the evolutionary states of these systems were discussed by
constructing the color-density diagram. We derived that the evolutionary states
of the two components of USPCBs show identical characteristics of other contact
binaries despite slower evolutionary status caused by smaller mass. We
suggested that both the fully convective limit claimed by Rucinski (1992) and
the AML theory recommended by Stepien (2006) can produce the short period
cut-off and a tertiary companion is very important during the formation of the
short period contact binaries by removing angular momentum from the host
eclipsing pairs under certain circumstances.
