Decoding the RVb enigma of DF Cyg: pulsations, circumstellar disks, and post-RGB evolution revealed by multi-wavelength observations

Abstract DF Cyg, an extensively studied RV Tauri star characterized by irregular pulsation dynamics, represents the sole RV Tauri star within the original Kepler field. This investigation integrates five decades of ground-based photometric data from the AAVSO, AFOEV, ASAS, ASAS-SN , and SuperWASP surveys with recently acquired TESS observations, conducting a multi-wavelength and multi-phase photometric analysis to elucidate the complex nature of DF Cyg. For the first time, we analyze approximately four years of Kepler space telescope data alongside TESS observations from sectors 14–15, 41, and 54–55. The precision of the TESS Sector 14 data approaches that of the Kepler dataset. Furthermore, TESS observations from sectors 15, 54, and 55, obtained in 120 s cadence mode, constitute the highest-precision photometric dataset for DF Cyg to date. By isolating long-term trends in the TESS data, we quantified short-term variations in the fundamental pulsation frequency and its integer harmonics. The two alternating short-term cycles in the TESS light curve facilitated the unambiguous identification of seven previously undetected integer harmonics of the fundamental frequency in the power spectrum (2f/9, 3f, 4f, 5f, 6f, 7f, and 8f), providing critical new insights into the star’s complex pulsation dynamics. A periodogram analysis of the combined ground- and space-based datasets revealed approximately 35 frequencies linked to both long- and short-term variability mechanisms. Stellar parameters derived from Gaia DR3 data—specifically, luminosity and radius estimates based on Type II Cepheid period-luminosity (PL) and period-radius (PR) relations—demonstrate consistency with values obtained through spectral energy distribution (SED) modeling. Complementary high-resolution spectroscopic analysis of DF Cyg, conducted using the SP_Ace code on McDonald Observatory data, yielded atmospheric parameters of T eff = 4781 K, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>g</mml:mi> <mml:mo>=</mml:mo> </mml:math> 1.74 cm s −2 , and [ Fe / H ] = − 0.07 dex, further constraining the star’s physical characteristics. This synthesis of multi-epoch, multi-instrument data advances the understanding of pulsational behavior and evolutionary status in RV Tauri systems. Our findings highlight DF Cyg as a critical benchmark for its class, as its characteristics bridge the gap between post-RGB systems in the Magellanic Clouds and higher-luminosity post-AGB stars.