Long-Term Orbital Period Behaviour of Contact Binaries V343 Ori and FZ Ori

In this study, we investigated orbital period variations of two contact binaries V343 Ori and FL Ori based on published minima times. Using the O-C analysis, it was found that both systems indicate orbital period increase. Mass transfer from less massive component to more massive component was used to explain increases in the orbital periods. On the other hand, the secular changes in their periods can be a sign of the thermal relaxation oscillation. In the O-C diagram of FL Ori, periodic variations also exist. Cyclic periodic changes can be explained as being the result of a light travel time effect via a tertiary body around the eclipsing pair. The minimum mass of probable tertiary component around FL Ori was found to be 0.63 M-circle dot. In addition, the cyclic variation may be evidence of magnetic activity of the components, which are late-type stars.