2025MNRAS.539..928S

The transiting long-period exoplanets are the most interesting follow-up targets for the next-generation instruments, using the most sophisticated observational techniques. However, the scarcity in their transit events often leads to larger uncertainties in their known ephemeris, also resulting in larger biases in their known physical properties in many cases. In this work, I have used the newer publicly available observations from TESS and CHEOPS for five very interesting long-period transiting exoplanets, i.e. HD95338 b, TOI-2134 c, K2-290 c, TOI-1898 b, and TOI-813 b, combined with the previously reported observations, to reanalyse their transit properties and estimate the updated ephemeris. The analyses also incorporated a critical noise treatment algorithm, which uses well-tested techniques such as wavelet denoising and Gaussian process regression, to effectively reduce the impact of various noise components present in the light curves on the estimated parameters. The study has resulted in a more precise estimation of ephemeris for all the targets, with the precision in the estimated periods being better than 5 s, except for TOI-813 b, for which the precision in the estimated period is better than 21 s. The other transit parameters also got updated, with statistically significant improvements seen in most of the cases, the major notable improvements being in the estimated value of the impact parameter of TOI-1898 b, the orbital semimajor axis of TOI-2134 c, and the radius of HD95338 b. Although long-period exoplanets are expected to show more significant transit timing variations in the presence of other undetected planetary-mass objects, no such variations were recorded for these targets.