tokens: of stars in the K2 samples prevent us from drawing any firm conclusion with regards to the slopes. As for the offsets, the effect of such differences seems to be at the level of ∼ ± 7 μ as at most. In addition, the fairly good agreement between SkyMapper and APOGEE does not exclude the possibility that the photometric T eff and [Fe/H] may be influenced differently in different fields (e.g. C3 and C6) by external factors such as, for instance, the extinction. A further point we would like to raise concerns the apparently larger scatter in the K2 fields. In that respect, we identify the order of magnitude of the Gaia and seismic ( PARAM ) parallax uncertainties (Fig. 12 ). The asteroseismic uncertainties are slightly larger in the case of K2. This can mainly be explained by the fact that the original mission, Kepler , continuously monitored stars for four years, whereas each campaign of K2 is limited to a duration of approximately 80 days ( Howell et al. 2014 ). Despite this, photometric systematics in the K2 data leading to, for instance, spurious frequencies are now well understood, and the pipelines used to produce light curves have been developed over time to remove or mitigate these effects. As a result, we do not expect the global seismic parameters to show any significant bias due to the K2 artefacts (see also Hekker et al. 2012 , and the Kepler and K2 Science Center website 5 ). Nonetheless, the astrometric uncertainties are also substantially larger for K2, almost doubled compared to Kepler . A possible reason for this would be that the regions around the ecliptic plane (such as C3 and C6) are observed less frequently, as a result of the Gaia scanning law, and also under less favourable scanning geometry, with the scan angles not distributed evenly ( Gaia Collaboration 2016b ). To test this hypothesis, we use the quantity visibility_periods_used : the number of visibility periods, that is a group of observations separated from other groups by a gap of at least four days, used in the astrometric solution. This way, we can assess if a source is astrometrically well-observed. This variable exhibits significantly higher values for Kepler , ranging from 12 to 17. The number of visibility periods for K2 are lower or equal to ten, indicating that the parallaxes could be more vulnerable to errors. The predicted uncertainty contrast between the Kepler and K2 fields is about a factor 6 of 1.6, which is indeed consistent with the location of the histogram peaks in the top panel of Fig. 12 . Fig. 12. Distribution of parallax uncertainties in Gaia DR2 ( top ) and in PARAM ( bottom ) for the Kepler (red), C3 (orange), and C6 (purple) fields. 5.2. Quasars and colour-magnitude diagram Lindegren et al. (2018) investigated the parallax offset using quasars, and obtained a global zero-point of about −30 μ as. It is no surprise that this value differs from the ones we obtain in the current work. Indeed, this parallax offset depends on magnitude and
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