SIMBAD references

The search volume-corrected period distribution of contact binaries of the W UMa type appears to reflect primarily the constant number ratio of ≃1/500 to the number of stars along the main sequence; there exist no evidence for angular momentum evolution. The maximum in contact binary numbers is located at shorter periods than estimated before, P ≃ 0.27 d. The drop in numbers towards the cut-off at P ≃ 0.215-0.22 d still suffers from the small number statistics while the cut-off itself remains unexplained. Only one out of seven short-period All-Sky Automated Survey variables with P < 0.22 d have been retained in the sample considered here within 8 < V < 13; this short-period field-sky record holder at P = 0.2178 d should be studied.