2020MNRAS.491.2760G

Only a few exoplanets are known to orbit around fast rotating stars. One of them is XO-6b, which orbits an F5V-type star. Shortly after the discovery, we started multicolour photometric and radial-velocity (RV) follow-up observations of XO-6b, using the telescopes of Astronomical Institute of the Slovak Academy of Sciences. Our main scientific goals were to better characterize the planetary system and to search for transit timing variations (TTVs). We refined several planetary and orbital parameters. Based on our measurements, the planet XO-6b seems to be about 10 per cent larger, which is, however, only about 2σ difference, but its orbit inclination angle, with respect to the plane of the sky, seems to be significantly smaller, than it was determined originally by the discoverers. In this case, we found about 9.5σ difference. Moreover, we observed periodic TTVs of XO-6b with a semi-amplitude of about 14 min and with a period of about 450 d. There are two plausible explanations of such TTVs: (1) a third object in the system XO-6 causing light-time effect, or (2) resonant perturbations between the transiting planet XO-6b and another unknown low-mass planet in this system. From the O-C diagram, we derived that the assumed third object in the system should have a stellar mass, therefore significant variations are expected in the RV measurements of XO-6. Since this is not the case, and since all attempts to fit RVs and O-C data simultaneously failed to provide a consistent solution, more realistic is the second explanation.