tokens: RR Lyrae stars that are likely Orphan members, and we obtained . This is very similar to the value reported by Sesar et al. (2013) and also significant 12 . We explored the existence of a metallicity gradient in the Orphan stream using our nonvariable and RR Lyrae sample 13 . The depiction of the metallicity versus φ 1 can be found in Fig. 5 . In both of our samples (nonvariable and RR Lyrae sample) we do not detect any significant correlation between the sky position φ 1 and metallicity. A similar outcome holds even when we include only stars with a high probability p ( A | B ) > 0.5 for both of our samples. One of the possible reasons for this discrepancy lies in the different metallicity calibrations between our study and Sesar et al. (2013) . In our case, we rely on the new calibration of the ΔS method using metallicities determined from high-resolution spectra ( Crestani et al. 2021 ), while Sesar et al. (2013) relied on the calibration of Layden (1994) which is slightly offset compared to metallicities obtained from high-resolution spectra (see, e.g., For et al. 2011 ; Chadid et al. 2017 ). Another reason could lie in the metallicity scale, where Sesar et al. (2013) values lie on the Zinn West (1984) scale 14 , while our metallicities are on a different metallicity scale ( Chadid et al. 2017 ; Sneden et al. 2017 ; Crestani et al. 2021 ). This could shift the metallicities of Sesar et al. (2013) toward the metal-rich end by up to 0.2 dex ( For et al. 2011 ). To conclude, using our dataset we were unable to confirm the existence of a metallicity gradient in the Orphan stellar stream. Fig. 5. Stream coordinates, φ 1 , versus [Fe/H] ( top panel ), and metallicity distribution function ( bottom panel ) for likely variable andnonvariable Orphan stream members. The color-coding of each nonvariable star represents the probability of association to the Orphan stream p ( A | B ), and the green squares represent RR Lyrae stars associated with the Orphan stream in our study. The histogram in the lower panel represents the metallicity distribution of the entire sample (black line) the RGB stars (red dashed line), BHB stars (blue dotted line), and RR Lyrae stars (green lines). The red solid line represents the kernel density estimate of the metallicity distribution of the RGB stars. 5.2. Grus II as a possible progenitor In the work by K19, the previously considered candidates for the Orphan progenitors, Segue 1 and UMa II ( Fellhauer et al. 2007 ; Newberg et al. 2010 ), were excluded based on their distance and proper motions. One viable candidate for the progenitor of the Orphan stream remained, Grus II, a UFD (found in the DES by Drlica-Wagner et al. 2015 ). Grus II matches with the Orphan stellar stream in the coordinates and proper motion space. Recently, line-of-sight velocities and chemical abundances became available for several stars associated with Grus II UFD ( Simon et al. 2020 ; Hansen et al. 2020 ). The line-of-sight velocities center
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