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In an era of extensive photometric observations, the catalogs of RR Lyr typevariable stars number tens of thousands of objects. The relation between theiron abundance [Fe/H] and the Fourier parameters of the stars light curveallows us to investigate mean metallicities and metallicity gradients invarious stellar environments, independently of time-consuming spectroscopicobservations. In this paper we use almost 6500 $V$- and $I$-band light curvesof fundamental mode RR Lyr stars from the OGLE-IV survey to provide a relationbetween the $V$- and $I$-band phase parameter $\varphi_{31}$ used to estimate[Fe/H]. The relation depends on metallicity, which limits its applicability. Weapply this relation to metallicity formulae developed for the Johnson $V$- andthe Kepler $Kp$-band to obtain the relation between [Fe/H] and $\varphi_{31}$for the $I$-band photometry. Last, we apply the new relation of Nemec to theOGLE-IV fundamental mode RR Lyr stars data and construct a metallicity map ofthe Magellanic Clouds. Median [Fe/H] is $-1.39\pm0.44$ dex for the LMC and$-1.77\pm0.48$ dex for the SMC, on the Jurcsik metallicity scale. We also finda metallicity gradient within the LMC with a slope of $-0.029\pm0.002$ dex/kpcin the inner 5 kpc and $-0.030 \pm0.003$ dex/kpc beyond 8 kpc, and no gradientin-between ($-0.019\pm0.002$ dex/kpc integrally). We do not observe ametallicity gradient in the SMC, although we show that the metal-rich RRabstars are more concentrated toward the SMC center than the metal-poor.

OGLE-ing the Magellanic System: Photometric Metallicity from Fundamental Mode RR Lyrae Stars