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Using the Hamilton Echelle Spectrograph at Lick Observatory, we have obtainedprecise radial velocities (RVs) of a sample of 373 G- and K-giant stars overmore than 12 years, leading to the discovery of several single and multipleplanetary systems. The RVs of the long-period (~53 years) spectroscopic binary$\epsilon$ Cyg (HIP 102488) are found to exhibit additional regular variationswith a much shorter period (~291 days). We intend to improve the orbitalsolution of the $\epsilon$ Cyg system and attempt to identify the cause of thenearly periodic shorter period variations, which might be due to an additionalsubstellar companion. We used precise RV measurements of the K-giant star$\epsilon$ Cyg from Lick Observatory, in combination with a large set of RVscollected more recently with the SONG telescope, as well as archival data sets.Our Keplerian model to the RVs characterizes the orbit of the spectroscopicbinary to higher precision than achieved previously, resulting in a semi-majoraxis of $a = 15.8 \mathrm{AU}$, an eccentricity of $e = 0.93$, and a minimummass of the secondary of $m \sin i = 0.265 M_\odot$. Additional short-period RVvariations closely resemble the signal of a Jupiter-mass planet orbiting theevolved primary component with a period of $291 \mathrm{d}$, but the period andamplitude of the putative orbit change strongly over time. Furthermore, in ourstability analysis of the system, no stable orbits could be found in a largeregion around the best fit. Both of these findings deem a planetary cause ofthe RV variations unlikely. Most of the investigated alternative scenarios,such as an hierarchical triple or stellar spots, also fail to explain theobserved variability convincingly. Due to its very eccentric binary orbit, itseems possible, however, that $\epsilon$ Cyg could be an extreme example of aheartbeat system.

Precise radial velocities of giant stars XV. Mysterious nearly periodic radial velocity variations in the eccentric binary $\epsilon$ Cygni