RW Dor: a newly formed shallow-contact binary with a possible substellar companion

The results of BV(RI)c CCD photometric monitoring of RW Dor, performed with the PROMPT-8 robotic telescope at CTIO in Chile from March 2015 to March 2017, are presented. The light curves exhibit variation with very weak Ou0027Connell effect and gradually change from symmetry to asymmetry. This variation can be explained by spot activities. By analysing with the Wilson-Devinney code, it is shown that RW Dor is a W-subtype shallow contact binary with a dark spot on the hotter and less massive component. In addition, the orbital period changes of the system are investigated by using thirteen new times of light minima together with those complied from the literature. A secular decrease dP/dt = -9.27x10^-9 d/yr is found to be superimposed on a cyclic oscillation (A_3 = 0.0058 days and P_3 = 95.38 yrs). The light-travel time effect due to the presence of a third component is the most plausible explanation of the cyclic period change. The minimum mass of the third body is estimated as M_3 ~ 0.06 M_Sun that is below the stable hydrogen-burning limit of M_3 ~ 0.072 M_Sun, and thus it may be a brown dwarf. Its orbital separation from the center of the system is less than 22.35 AU. Both the shallow-contact configuration and the long-term period decrease may suggest that RW Dor is a new formed contact binary via a Case A mass transfer.