# 7.4.3 Calibration models

The SOS Cep&RRL processing uses tools such as: period-amplitude ($PA$) and period-luminosity ($PL$) relations in the $G$-band, as described in the documentation for the processing of RR Lyrae and Cepheid stars released in Gaia Data Release 1, (Clementini et al. 2016). For the Gaia Data Release 2 data processing (Clementini et al. 2019) we also use tools based on the $G_{\rm BP}$ and $G_{\rm RP}$ photometry, such as the period-luminosity in the RP-band and the period-Wesenheit ($PW$) relation in $G$, $G_{\rm RP}$. Furthermore, we implemented i) use of parallaxes according to the Astrometric Based Luminosity formulation, (i.e. working directly in parallax space; see, e.g., (Gaia Collaboration et al. 2017) and references therein) and applying different $PL$, $PW$ relations depending on source position on sky (whether in the Large Magellanic Cloud, in the Small Magellanic Cloud or outside them); ii) calculation of metallicity ([Fe/H]) for the RR Lyrae stars and for $\delta$ Cepheid variables with period shorter than about 6 days from the Fourier parameters and, iii) calculation of interstellar absorption in the $G$-band for the RRab stars from a relation based on $G$-band peak-to-peak amplitude and period.