Gaia Data Release 1: Testing parallaxes with local Cepheids and RR Lyrae stars

Parallaxesfor 331 classical Cepheids, 31 Type II Cepheidsand 364 RR Lyrae stars in common between Gaia and the Hipparcos and Tycho-2 cataloguesare published in Gaia Data Release 1 (DR1) as part of the Tycho-Gaia Astrometric Solution (TGAS). In order to test these first parallaxmeasurements of the primary standard candlesof the cosmological distance ladder, that involve astrometrycollected by Gaia during the initial 14 months of science operation, we compared them with literature estimates and derived new period-luminosity ($PL$), period-Wesenheit ($PW$) relations for classical and Type II Cepheidsand infrared $PL$, $PL$-metallicity ($PLZ$) and optical luminosity-metallicity ($M_V$-[Fe/H]) relations for the RR Lyrae stars, with zero points based on TGAS. The new relations were computed using multi- band($ V,I,J,K_{\mathrm{s}},W_{1}$) photometry and spectroscopic metal abundances available in the literature, and applying three alternative approaches: (i) by linear least squaresfitting the absolute magnitudesinferred from direct transformation of the TGAS parallaxes, (ii) by adopting astrometric-based luminosities, and (iii) using a Bayesian fitting approach. TGAS parallaxesbring a significant added value to the previous Hipparcos estimates. The relations presented in this paper represent first Gaia-calibrated relations and form a "work-in-progress" milestone report in the wait for Gaia-only parallaxesof which a first solution will become available with Gaia's Data Release 2 (DR2) in 2018.