V405 Draconis: A Deep Contact Binary with a Tertiary Companion and a Quadruple System Candidate

We continued our analysis of the O-C diagram of the EW-type eclipsing binary V405 Dra using recently available data. We refined the orbital parameters of the third body responsible for the short-period oscillations in the O-C curve, obtaining P 3=513.12(20) d, a 12 i 3=0.3862(15) AU, and e 3=0.1690(48). Assuming a total mass of 1.56 M for the eclipsing binary, the minimum mass of the third body is estimated to be about 0.5 M. We also investigated the complex long-term O-C trend and found that it may be partially explained by the light-time effect caused by a possible fourth body, with a characteristic period of about 23.5 yr and a minimum mass of about 0.9 M. However, the fourth-body model cannot fully reproduce the observed long-term variation, particularly the earliest data, and the nature of part of the long-term trend remains unexplained.