Giant Outer Transiting Exoplanet Mass (GOTEM) Survey.VII. TOI-6041: a multi-planet system including a warm Neptune exhibiting strong TTVs

We present the characterization of the TOI-6041 system, a bright ($V=9.84\pm 0.03$) G7-type star hosting at least two planets. The inner planet, TOI-6041b, is a warm Neptune with a radius of $4.55_{-0.17}^{+0.18}{R}_{\oplus }$, initially identified as a single-transit event in $\mathit{\text{TESS}}$ photometry. Subsequent observations with $\mathit{\text{TESS}}$ and $\mathit{\text{CHEOPS}}$ revealed additional transits, enabling the determination of its $26.04945_{-0.00034}^{+0.00033}$~d orbital period and the detection of significant transit timing variations (TTVs), exhibiting a peak-to-peak amplitude of about 1~hour. Radial velocity (RV) measurements obtained with the APF spectrographs allow us to place a $3\sigma$ upper mass limit of $28.9M_{\oplus }$ on TOI-6041b. In addition, the RV data reveal a second companion, TOI-6041c, on an 88~d orbit, with a minimum mass of $0.25M_{\mathrm{J}\mathrm{u}\mathrm{p}}$. A preliminary TTV analysis suggests that the observed variations could be caused by gravitational perturbations from planet c; however, reproducing the observed amplitudes requires a relatively high eccentricity of about 0.3 for planet c. Our dynamical stability analysis indicates that such a configuration is dynamically viable and places a $1\sigma$ upper limit on the mass of TOI-6041c at $0.8M_{\mathrm{J}\mathrm{u}\mathrm{p}}$. An alternative is the presence of a third, low-mass planet located between planets b and c, or on an inner orbit relative to planet b – particularly near a mean-motion resonance with planet b – which could account for the observed variations. These findings remain tentative, and further RV and photometric observations are essential to better constrain the mass of planet b and to refine the TTV modeling, thereby improving our understanding of the system’s dynamical architecture.