SX Aurigae: A close binary at the early stage of contact phase

•New multi-band CCD light curves of SX Aur have been obtained.•Orbital period changes were investigated and were found to exhibit a secular increase.•From analysis of light and radial velocity curves, it was determined that SX Aur is a geometrical contact binary.•Absolute parameters of the components and orbital properties of the system were estimated.

This study presents analysis of new multi-band light curves of β Lyrae-type eclipsing binary SX Aur. Radial velocities of both components of the system were collected from the literature and simultaneous light curve solutions made to determine the properties of the system and absolute elements of its components. The derived masses and radii of the components are M1 = 11.3 ± 0.2 M⊙, M2 = 6.9 ± 0.1 M⊙, R1 = 5.32 ± 0.02 R⊙ and R2 = 4.23 ± 0.03 R⊙, while the distance of the system is estimated at 1220 ± 75 pc. It was found that SX Aur is a shallow-contact binary and thus may be in the stage between β Lyrae and W UMa-type binaries. From O–C based period analysis of SX Aur, it is seen that the period of the system increases at a rate of 0.0055 s yr−1. The most plausible physical mechanism for this period variation is non-conservative mass transfer from the secondary to the primary component at a rate of 3.13 × 10−7 M⊙ yr−1.