LY Aurigua: A mass-transferring O-type contact binary with a tertiary stellar companion

LY Aur is a contact massive close binary with a period of a little more than four days. The first O-C analysis of this early-type binary presented in this paper suggests that the period of the system is increasing continuously at a rate of dP/dt=+7.2×10-7 days/year, while a cyclic oscillation with the period of 12.5 years is obvious. The long-term increasing can be explained by mass transfer from the less massive companion to the more one on the nuclear time-scale of less massive body, which suggests that the contact configuration will be broken and this binary will evolve into a semi-detached system. The periodic oscillation may be the consequence of the light-travel time effect of the third body, whose mass is no less than 3.4 M⊙. It is expected that the third body may play an important role for the origin and evolution of the system by removing angular momentum from the central system, making the eclipsing pairs to have a low angular momentum, while initially it may have had a longer orbital period, with larger angular momentum. The original system may have evolved into the present contact configuration via a case A mass transfer.