Bi-site time-series photometric and high-resolution spectroscopic observations were made for the double-mode high-amplitude δ Scuti star VX Hya. The fundamental frequency f ₀ = 4.4763 c days⁻¹, the first overtone f ₁ = 5.7897 c days⁻¹, and 23 harmonics and linear combinations of f ₀ and f ₁ are detected by pulsation analysis. From the spectroscopic data, we get [Fe/H] = −0.2 ± 0.1 dex. The period change rate of the fundamental mode is obtained by using the Fourier-phase diagram method, which gives the value of . With these results from the observations, we perform theoretical explorations with the stellar evolution code MESA, and constrain the models by fitting f ₀, f ₁, and (1/P ₀)(dP ₀/dt) within 3σ deviations. The results show that the period change of VX Hya could be ascribed to the evolutionary effect. The stellar parameters of VX Hya could be derived as: the mass 2.385 ± 0.025 M _⊙, the luminosity log(L/L _⊙) = 1.93 ± 0.02, and the age (4.43 ± 0.13) × 10⁸ years. VX Hya is found to locate at the post-main-sequence stage with a helium core and a hydrogen-burning shell on the H–R diagram.

In this work, the photometric data from the American Association of Variable Star Observers are collected and analyzed on the SX Phoenicis star DY Pegasi (DY Peg). From the frequency analysis, we get three independent frequencies: f ₀ = 13.71249 c d⁻¹, f ₁ = 17.7000 c d⁻¹, and f ₂ = 18.138 c d⁻¹, in which f ₀ and f ₁ are the radial fundamental and first overtone mode, respectively, while f ₂ is detected for the first time and should belong to a nonradial mode. The O − C diagram of the times of maximum light shows that DY Peg has a period change rate (1/P ₀)(dP ₀/dt) = −(5.87 ± 0.03) × 10⁻⁸ yr⁻¹ for its fundamental pulsation mode, and should belong to a binary system that has an orbital period P _orb = 15425.0 ± 205.7 days. Based on the spectroscopic information, single star evolutionary models are constructed to fit the observed frequencies. However, some important parameters of the fitted models are not consistent with that from observations. Combing the information from observation and theoretical calculation, we conclude that DY Peg should be an SX Phoenicis star in a binary system and accreting mass from a dust disk, which was the residue of its evolved companion (most probably a hot white dwarf at the present stage) produced in the asymptotic giant branch phase. Further observations are needed to confirm this inference, and it might be potentially a universal formation mechanism and evolutionary history for SX Phoenicis stars.

Stellar structure and evolution theory is one of the bases in modern astronomy. Stellar inner structures and their evolutionary states can be precisely tested by asteroseismology, since the inner information is brought to the stellar surface by the global oscillating waves and becomes observable. For stellar evolutionary speed (i.e., how long timescale does a star stay at a special evolution phase?), because of the insurmountable gap between the timescales of the evolutionary history of human civilization and a star, it can only be roughly tested by ensemble of stars in different evolutionary stages in most cases, and all the snapshots of these stars make up our global view of stellar evolution. The effect of stellar evolution on the structure and the corresponding global size of a pulsating star will lead to tiny period variations of its pulsation modes, which are the most valuable indicators of its evolutionary state and can be used to test the stellar evolution theory by a single star rather than ensemble of stars. Here, we report a High-Amplitude δ Scuti star AE Ursae Majoris, which is located in the post main-sequence (MS) evolutionary stage and its observed linear period variation rate can be practically ascribed to its evolutionary effect. The result tests the stellar evolution theory from the pre-MS to post-MS with an unprecedented precision by a single star, and the framework can be extended to other type of pulsating stars to perform precise evolutionary asteroseismology, which aims to test the current stellar evolution theory in different evolutionary stages, discover the discrepancies between the theory and observations, and ultimately build a complete and precise stellar evolution theory to backtrack the history of each of these stars.

High-amplitude δ Scuti stars (HADS) that pulsate in their first three radial modes are rare in current samples. Here, we analyse five such triple-mode HADS observed by the Transiting Exoplanet Survey Satellite (TESS) and report that the previously considered second overtone mode (f₂) is actually the non-radial component of a resonating integration mode (RI mode, resulting from resonance between a radial p-mode and a non-radial p-g mixed mode), which shows significant amplitude and frequency variations over short timescales (approximately 20 days). This RI mode appears to be widespread among these stars. Notably, all five stars are in the post-main sequence evolutionary phase, actively crossing the Hertzsprung gap. These stars offer valuable insights into stellar evolution during the Hertzsprung gap, which is one of the most rapid evolutionary stages in a star’s life.