# The spectroscopic binaries RV Tauri and DF Cygni

## TL;DR

This study confirms the binary nature of RV Tauri stars RV Tau and DF Cyg, derives their orbital parameters, and links their long-term variability to binarity and circumbinary discs, advancing understanding of their evolution.

## Contribution

The paper provides the first detailed orbital solutions for RV Tau and DF Cyg, demonstrating their binary status and the influence of their orbits on observed variability.

## Key findings

- Both stars are confirmed as binaries with long orbital periods.
- Long-term photometric variability is explained by orbital motion and circumbinary discs.
- Spectroscopic evidence of outflows related to the companions was observed.

## Abstract

Aim: The focus of this paper is on two famous but still poorly understood RV Tauri stars: RV Tau and DF Cyg. We aim at confirming their suspected binary nature and deriving their orbital elements to investigate the impact of their orbits on the evolution of these systems. This research is embedded into a wider endeavour to study binary evolution of low- and intermediate-mass stars. Method: The high amplitude pulsations were cleaned from the radial-velocity data to better constrain the orbital motion. We used Gaia DR2 parallaxes in combination with the SEDs to compute their luminosities which were complemented with the ones computed using a period-luminosity-colour relation. The ratio of the circumstellar infrared flux to the photospheric flux obtained from the SEDs was used to estimate the orbital inclination of each system. Results: DF Cyg and RV Tau are binaries with spectroscopic orbital periods of 784$\pm$16 days and 1198$\pm$17 days, respectively. These orbital periods are found to be similar to the long-term periodic variability in the photometric time series, indicating that binarity indeed explains the long-term photometric variability. Both systems are surrounded by a circumbinary disc which is grazed by our line-of-sight. As a result, the stellar photometric flux is extinct periodically with the orbital period. Our derived orbital inclinations enabled us to obtain accurate companion masses for DF Cyg and RV Tau. Analysis of the Kepler photometry of DF Cyg revealed a power spectrum with side lobes around the fundamental pulsation frequency. This modulation corresponds to the spectroscopic orbital period and hence to the long-term photometric period. Finally we report on the evidence of high velocity absorption features related to the H$_{\alpha}$ profile in both objects, indicating outflows launched from around the companion.