KIC 2831097 times of maximum brightness

The observations were programmed at the predicted quadratures of the orbit, when the two mean radial velocities should differ by about 100km/s. The GEOS collaboration complemented the spectroscopic survey by a photometric one. We also analysed Gaia time series and intensive TESS photometry.

The radial velocity curves obtained at the quadratures show the same mean radial velocity (-203km/s), ruling out the possibility of a LTTE. The constant mean radial velocity, the [Fe/H] content determined from ground-based CCD observations and the astrometric parameters supplied by Gaia allow us to infer that KIC 2831097 is a single, high-velocity, metal-poor RRc star belonging to the galactic halo. We revisited Kepler photometry, detecting a weak Blazhko effect consisting in oscillation of only 1.1% of the period, in about 50 d.

We also analysed the TESS photometry of Kepler-1601, whose photometry is contaminated by KIC 2831097. We collected 116 maxima, from the ground-based campaign, 103 maxima from TESS, and 4 maxima from Gaia. In total, we have 3624 times of maximum brightness.

Linear ephemerides cannot fit the whole dataset, but only parts of them. The period shows a tendency to decrease in value, as if it were an evolutionary effect, but not at a constant rate. The spectroscopic and photometric campaigns performed in the framework of a professional-amateur project failed to confirm that KIC 2831097 belongs to a binary system: the chase remains open.

As an RR Lyr variable, KIC 2831097 shows an intriguing evolution of the period.