Contact binaries at the short period cut-off - I. Statistics and the first photometric investigations of 10 totally eclipsing systems

The period distribution of contact binaries exhibits a very sharp short period cut-off at 0.22 d. In order to provide valuable information on this short period limit, we observed 10 totally eclipsing contact binaries with orbital periods near this cut-off. By detailed analysis using the W-D code, we determined that two of these systems are A-subtype contact binaries while the others areW-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 ultrashort period contact binaries (USPCBs) was carried out, and physical parameters of 55 USPCBs were obtained. Some common properties for these systems were derived. Due to the study of the period-colour diagram of USPCBs, we found that the period-colour 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 colour-density diagram. We derived that the evolutionary states of the two components of USPCBs show identical characteristics of other contact binaries despite a slower evolutionary status caused by smallermass. We suggested that both the fully convective limit claimed by Rucinski and the angular momentum loss theory recommended by Stepien 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.