A Unique Low-mass-ratio Contact Eclipsing Binary System under the Period Cutoff

We present a multiband photometric analysis of CRTS J163819.6+03485, the first low-mass-ratio contact binary system with a period of 0.2053321 day under the contact binary period limit. The unprecedented combination of mass ratio and period makes this system unique for eclipsing binary (EB) research. Using new multiband photometric observations, we explored the parameter space of this unique total EB system through a detailed scan in the mass ratio–inclination plane and using the pikaia genetic algorithm optimizer. The best set of relative physical parameters and corresponding uncertainties was adopted through Markov Chain Monte Carlo sampling of the parameter space. The resulting mass ratio of the system is q = 0.16 ± 0.01. The absolute parameters were derived by adopting an empirical mass–luminosity relation. Period changes are also investigated by using new observations and archival photometric light curves from massive astronomical surveys, which revealed in a preliminary solution the presence of a possible low-mass tertiary companion. The origin and evolutionary status of the system are investigated through the detached binary formation scenario.