Long-term monitoring of WASP-19 b: Signs of apsidal precession and molecular signatures

Context. With over 6000 exoplanets discovered to date, approximately 12 % are classified as hot-Jupiters. Due to their large sizes and short orbital periods (P < 10 day), they are easier to detect and provide crucial insights into planetary formation, atmospheric properties, and orbital dynamics. Among these, ultra-short-period exoplanets (P ≤ 1 d) are particularly interesting, as they are expected to undergo orbital decay driven by strong tidal interactions. Despite theoretical predictions, WASP-12 b and WASP-4 b remain the confirmed hot-Jupiters experiencing measurable orbital decay. Aims. This study presents a homogeneous analysis of WASP-19 b to investigate both its orbital dynamics and atmospheric composition. Leveraging a 15-year dataset, our goal is to assess whether the system exhibits long-term deviations from a constant orbital period and to investigate whether any detected variations are consistent with tidal orbital decay, apsidal precession, or periodic signals indicative of a potential planetary perturber. Additionally, we also construct a photometric transmission spectrum to characterize its atmosphere. Methods. We analyze multi-wavelength light curves, incorporating starspot modeling with PRISM to account for stellar inhomogeneities. To assess orbital evolution, we fit linear, quadratic, and cubic ephemeris models to transit timing residuals with respect to a non-decaying orbit. Results. Our analysis, which includes 27 new transits, reveals no statistically significant periodic signal in the transit timings. Although none of the tested ephemeris models fully reproduce the observed timing scatter, the mid-transit times exhibit systematic deviations from a strictly constant orbital period and are best reproduced by the cubic ephemeris in a relative model-comparison sense, indicating a slow, non-periodic long-term trend over the ~ 15-year baseline. This behavior is more consistent with gradual orbital precession than with monotonic tidal decay, for which a dominant quadratic trend would be expected. Fitting a precession model yields a rate of ω̇obs = (1.00 ± 0.12) × 10−4 rad/orbit, corresponding to a planetary Love number k2p = 0.107 ± 0.08, in agreement with recent independent estimates. The transmission spectrum reveals signatures of Na, K, and H2O, with no strong evidence of TiO/VO, likely due to the resolution limits of the photometric data. Conclusions. Our results support that apsidal precession could be the dominant process governing the long-term orbital evolution of WASP-19b, possibly sustained by weak eccentricity forcing from the wide companion WASP-19 B. These orbital dynamics can, in turn, impact the atmospheric structure by modulating the irradiation history, potentially altering molecular abundances over time. Our findings highlight the importance of combining TTV analyzes with multi-wavelength atmospheric data, while emphasizing that additional high-quality timing and spectroscopic observations are required to corroborate the fidelity of the proposed orbital model.