Photometric study of three ultrashort-period contact binaries

We carried out high-precision photometric observations of three eclipsing ultrashortperiod contact binaries (USPCBs). Theoretical models were fitted to the light-curves by means of the Wilson-Devinney code. The solutions suggest that the three targets have evolved to a contact phase. The photometric results are as follows: a) 1SWASP J030749.87-365201.7, q=0.439\pm0.003, f=0.0\pm3.6%; b) 1SWASP J213252.93-441822.6, q=0.560\pm0.003, f=14.2\pm1.9%; c) 1SWASP J200059.78+054408.9, q=0.436\pm0.008, f=58.4\pm1.8%. The light curves show O'Connell effects, which can be modeled by assumed cool spots. The cool spots models are strongly supported by the night-to-night variations in the I-band light curves of 1SWASP J030749.87-365201.7. For a comparative study, we collected the whole set of 28 well-studied USPCBs with P < 0.24 day. Thus, we found that most of them (17 of 28) are in shallow contact (i.e. fill-out factors f<20%). Only 4 USPCBs have deep fill-out factors (i.e. f>50%). Generally, contact binaries with deep fill-out factors are going to merge, but it is believed that USPCBs have just evolved to a contact phase. Hence, the deep USPCB 1SWASP J200059.78+054408.9 seems to be a contradiction, making it very interesting. Particularly, 1SWASP J030749.87-365201.7 is a zero contact binary within thermal equilibrium, implying that it should be a turn-off sample as predicted by the thermal relaxation oscillation (TRO) theory.