Sebastián Zúñiga-Fernández

Short-period white-dwarf–main-sequence (WDMS) binaries provide direct constraints on the outcomes of common-envelope (CE) evolution and the survival of low-mass companions during engulfment. We report the discovery and confirmation of 10 totally eclipsing WD + cool-M-dwarf binaries identified through a search tailored to minute-scale eclipses in Transiting Exoplanet Survey Satellite (TESS) high-cadence data and validated with SPECULOOS multiband photometry. The systems have orbital periods of a few hours and companion masses of 0.094–0.175 M _⊙ . Our pilot search of ∼37,000 Gaia-selected WDs increases the number of WD+M systems first identified through TESS by an order of magnitude, demonstrating that minute-scale WD eclipses occupy a previously underexplored discovery regime for TESS. SPECULOOS follow-up establishes the WDs as the sources of the eclipse signals, while chromatic eclipse dilution and broadband spectral-energy-distribution (SED) modeling provide complementary constraints on the component properties. These systems extend the TESS-discovered eclipsing WD+M population toward lower companion masses and establish an effective, scalable framework for expanding the search from the ∼37,000 WDs analyzed here to a pool of O (10 ⁶ ) Gaia WD candidates with TESS photometry.

White dwarf–brown dwarf (WD+BD) binaries are rare laboratories for probing both substellar survival through post-main-sequence evolution and the physics of strongly irradiated atmospheres. We present time-resolved eclipse photometry and radial-velocity confirmation of J0404+1112, a compact ( P = 2.93 hr), totally eclipsing WD+BD system initially reported in Research Notes of the AAS by J. de Wit et al. New high-cadence observations resolve the total eclipse of the WD and, combined with spectroscopy, broadband photometry, and the Gaia parallax, yield a revised system architecture. We find a hot DA WD with T _eff ≃ 28,000 K, log g ≃ 8.0 , M _WD ≃ 0.6 M _⊙ , and R _WD ≃ 0.015 R _⊙ , orbited by a ∼40 M _Jup BD with radius ∼0.9 R _Jup . These parameters supersede earlier spectral energy distribution (SED)-based estimates in which the WD temperature, radius, distance, and extinction were strongly covariant. We break this degeneracy with normalized Balmer-line profile fitting and high-cadence eclipse photometry that constrains the white dwarf radius geometrically. The total eclipses enable isolation of the brown dwarf (BD) flux and constrain its nightside emission. With its short orbital period and strong irradiation, J0404+1112 provides a uniquely compact, high-sensitivity benchmark for probing atmospheric heat redistribution and photochemistry in the temperature regime of the hottest ultrahot Jupiters, spanning substellar-point and nightside temperatures of ∼3700 K and ∼1750 K within a single ∼3 hr orbit.

Context. The young hierarchical quadruple system HD 98800 is composed of the tight binaries AaAb and BaBb on a wide, highly inclined outer orbit. The B subsystem hosts a circumbinary disc in a polar configuration, and the geometry of the system offers a rare opportunity to observe the passage of the disc around BaBb in front of AaAb. Aims. We update the overall orbital solution to offer precise time windows of the main occultation features by combining the orbit based on the most recent knowledge of the disc structure. Methods. We combined new and published radial velocity measurements and multi-wavelength astrometry for the outer AB orbit and the two inner subsystems, AaAb and BaBb, in a joint orbital fit. Results. The revised solution is consistent with previous dynamical masses and improves the outer orbit, reducing the uncertainties in the period and periastron epoch by about a factor of two. It also narrows the predicted crossing-phase windows to 5–15 days at the 1 σ level, improving the timing of ingress, egress, and the first cavity-crossing predictions. Conclusions. These results provide a more accurate timing framework for future observations of the occultation, although the predicted epochs remain model-dependent because of uncertainties in the disc structure.

Binary stars are ubiquitous; yet it remains unclear how wide-orbit stellar companions influence the formation of hot Jupiters, particularly around M dwarfs. Here, we first report the discovery of TOI-5628Ab, a giant planet transiting a midtype M dwarf ( M _* = 0.36 ± 0.02 M _⊙ ) every 4.34 days, accompanied by an associated white dwarf TOI-5628B ( M _WD = 0.59 ± 0.16 M _⊙ ) at a projected distance of about 2500 au. Using TESS, ground-based photometry, and SPIRou radial velocities, we constrain the planet radius to 0.74 ± 0.04 R _J and mass to 0.09 ± 0.04 M _J , with a 3 σ upper limit of 0.22 M _J . Building on this system, we further conduct a homogeneous systematic search for comoving stellar companions with projected semimajor axes between 100 and 10,000 au around all M dwarfs with confirmed giant planets with periods smaller than 10 days and radii larger than 0.7 R _J , as well as a group of field M stars with stellar properties similar to the planet sample, based on the stellar kinematics from Gaia DR3. We measure a stellar multiplicity of 34.2% ± 9.5% for M dwarfs hosting short-period giant planets, which is substantially higher than the fraction of 5.3% ± 3.7% for the field M stars by approximately a factor of 6. Our results suggest that wide-orbit stellar companions tend to promote the formation of short-period gas giants around M stars with masses 0.21 ≤ M _* ≤ 0.64 M _⊙ , and high-eccentricity migration may play an important role in producing such systems.

Context. Most long gamma-ray bursts (GRBs) exhibit afterglows that are broadly consistent with external forward-shock emission, which is typically described by smooth broken power-law decays in the multiband light curve. However, a minority of well-sampled GRBs deviates from this behavior, including the GRB we investigated, GRB 250129A. This source shows multiple late-time rebrightenings at X-ray and optical wavelengths. Rebrightenings are often attributed to energy injection from prolonged activity of the central engine, refreshed shocks from delayed shell collisions, density jumps in the ambient medium, or to the angular jet structure and viewing-angle effects. Aims After a comprehensive analysis of the prompt emission of the GRB observed in gamma rays and in the near-infrared, we investigated the physical origin of the multiple X-ray and optical flaring episodes observed in GRB 250129A. Methods. We conducted comprehensive temporal and spectral multiband analyses of GRB 250129A. The physical processes at play in the afterglow light curves were investigated using several methods, ranging from empirical fitting to Bayesian inference. The high-quality monitoring of the flare episodes, together with the connection between the prompt emission and the afterglow, enabled us to test the consistency of the fireball model and to constrain, reject, or confirm alternative scenarios. Results. GRB 250129A is an interesting GRB with multiwavelength prompt and afterglow emission. By conducting the prompt and time-resolved analyses, we obtained an isotropic-equivalent energy of E _{iso,  γ} = (1.35 ± 0.12)×10 ⁵³ erg. By modeling the afterglow using an agnostic Bayesian framework ( NMMA ), we ruled out a single external-shock evolution and a one-time energy-injection scenario. By further performing numerical calculations, we demonstrated that the rebrightening episodes are consistent with refreshed shocks arising from delayed collisions between relativistic shells. This agrees with evolving outflow dynamics. Conclusions. Based on the consistency between our analyses of the prompt and afterglow GRB 250129A data, together with prior general knowledge on microphysical properties, we find that two statistically significant rebrightening episodes occurred within 1.1 days post trigger and can be explained by a sequence of refreshed shocks. We stress that the availability of temporally and spectrally rich GRB datasets, such as the one we present here, provides a powerful means to test current modeling frameworks.

Context . A particularly intriguing subclass of rocky exoplanets are ultra-short-period worlds, which orbit their host stars in less than a day. These planets are particularly rare around M dwarf stars, and only ten of them have a constrained mass and radius so far. Aims . We present the validation and characterisation of the ultra-short-period (0.3 days) Earth-sized planet TOI-4552 b orbiting a nearby (27.26 pc away) M4.5V dwarf. Methods . We complemented TESS photometry ground-based transit observations from LCO, ExTrA, and SPECULOOS to validate the planetary radius, and we cleared the field of any contaminants. Speckle imaging with Zorro (Gemini-S) rules out false-positive scenarios caused by eclipsing binary sources. Spectroscopic observations with NIRPS and HARPS were used to obtain stellar abundances, constrain the planetary mass, and estimate the orbital parameters in conjunction with the transit observations. Results . TOI-4552 is a quiet star. It lacks short-term stellar variations in photometric or radial velocity data that might be associated with stellar rotation. Long-term photometric data from ASAS-SN also suggest a lack of activity signals. TOI-4552 b ( M _p = 1.83 ± 0.47 M _⊕ , R _p = 1.11 ± 0.04 R _⊕ ) lies between the Earth-like and iron-rich composition tracks on the mass-radius diagram. The exopie interior structure model, without constraints from refractory abundance ratio, yields a core mass fraction of 0.54 _−0.25 ^+0.17 and a bulk density of 7.74±2.14g/cm ³ . Since the core mass fraction spans a wide range because the uncertainty on the mass is high, the definitive interior composition cannot be determined with the current dataset. Conclusions . TOI-4552 b probably is marginally richer in iron than the Earth, but confirmation of its status requires additional precise radial velocity measurements. Combined with its high emission spectroscopic metric (ESM = 19.5), negligible stellar activity, and short orbital period, TOI-4552 b emerges as a compelling target for atmospheric and surface composition studies with JWST.

We report the validation of a third terrestrial exoplanet in the nearby (22 pc) TOI-2267 system. TOI-2267 is a binary system with stellar components TOI-2267A (M5, 3030 K) and TOI-2267B (M6, 2930 K), with an on-sky separation of 0 . ″ 384 (8 au projected separation). TOI-2267 hosts two Earth-sized planets (TOI-2267 b, 1.00 ± 0.11 R _⊕ , and TOI-2267 c, 1.14 ± 0.13 R _⊕ , if orbiting the primary star; or 1.22 ± 0.29 R _⊕ and 1.36 ± 0.33 R _⊕ , respectively, if orbiting the secondary star) with orbital periods of 2.3 and 3.5 days. This system also contains a third Earth-sized planet candidate with an orbital period of 2.0 days that was previously identified as a likely planet with a low false-positive probability, but could not be firmly validated due to the lack of independent observations beyond TESS data. We combine two new transit observations from the 5.1 m Hale Telescope at Palomar Observatory with archival TESS data and high-resolution imaging to statistically validate the planetary nature of TOI-2267 d (0.98 ± 0.09 R _⊕ if orbiting the primary star, or 1.77 ± 0.43 R _⊕ if orbiting the secondary star) using the updated TRICERATOPS+ pipeline. We attempt to determine the host star for TOI-2267 d using a transit shape stellar density analysis, but are unable to conclusively assign a host. Our validation of TOI-2267 d suggests that TOI-2267 either is the first known double-transiting M-dwarf binary system or hosts three planets in an extremely compact orbital configuration.