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tokens: of Gaia’s astrometry. The V -band bolometric correction of − 0.847 ± 0.075 was derivedusing the polynomial equations from Flower (1996) combined with the coefficients and the solar bolometric magnitude from Torres (2010) . Assuming no reddening ( A v = 0 mag), we found a stellar radius of R ⋆ = 0.622± 0.051 R ⊙ , which agrees with the spectroscopic radius derived using SpecMatch-Emp (cf. Table 2 ). We finally converted T eff , R ⋆ , and [Fe/H] into stellar mass M ⋆ and surface gravity log g ⋆ using the empirical relations of Mann et al. (2015) and Monte Carlo simulations. We found that GJ 9827 has a mass of M ⋆ = 0.637 ± 0.051 M ⊙ and a surface gravity of log g ⋆ = 4.657 ± 0.042 (cgs), which agrees with the spectroscopic gravity derived using SME (cf. Table 2 ). According to the analysis performed with SpecMatch-Emp , the three stars 2 whose spectra best match the HARPS and HARPS-N spectra of GJ 9827 have masses between 0.62 and 0.64 M ⊙ , confirming our results. The derived stellar mass and radius are are given in Table 1 . 3.3 Stellar activity and rotation period The K2 light curve of GJ 9827 displays a quasi-periodic photometric variability with a peak-to-peak amplitude of about 0.4 % ( Fig. 2 , left panel). Given the late spectral type of the star (K6 V), the observed photometric variation is very likely caused by active regions (sun-like spots and plages) crossing the visible stellar hemisphere as the star rotates about its axis. This is corroborated by the detection of emission components in the cores of the Ca II H K lines ( Fig. 3 ), from which we measured an average S-index of 0.677 ± 0.034 and 0.739 ± 0.021 using the HARPS and HARPS-N spectra, respectively. Fig. 2 Left panel : Gaussian process regression model applied to the K2 light curve. The black points are the K2 light curve, and the purple curve and shaded band are the GP model predictive mean and 1-sigma uncertainty. Right panel : K2 light curve folded to the P rot ∕2 stellar rotational period. Fig. 3 Cores of the Ca II H and K lines of GJ 9827 as observed with HARPS. Applying the auto cross-correlation technique to the K2 light curve, Niraula et al. (2017) and Rodriguez et al. (2018) found that the rotation period P rot of the star is either ~17 or 30 days. A Gaussian process (GP) analysis of the K2 light curve (Sect. 5.2 ) yields a bimodal posterior distribution with rotational periods peaking at 15.1 ± 1.6 and 30.7 ± 1.4 days, and thus does not provide a conclusive result about the rotation period of the star. However, we note that the ratio between the two measurements is close to 2, suggesting that the first might be the harmonic of the second. A visual inspection of the K2 time series photometry reveals that there are two dips whose minima occur at BJD TBD -2454833 ≈ 2922 and 2971 days, with a duration of ~20 and 16 days, respectively ( Fig. 2 , left panel). If the observed dips are caused by active regions crossing the visible hemisphere of GJ 9827, the rotation period