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We present a chemo-dynamical study of the Orphan stellar stream using acatalog of RR~Lyrae pulsating variable stars for which photometric,astrometric, and spectroscopic data are available. Employing low-resolutionspectra from the Sloan Digital Sky Survey (SDSS), we determined line-of-sightvelocities for individual exposures and derived the systemic velocities of theRR~Lyrae stars. In combination with the stars' spectroscopic metallicities and\textit{Gaia} EDR3 astrometry, we investigated the northern part of the Orphanstream. In our probabilistic approach, we found 20 single mode RR~Lyraevariables likely associated with the Orphan stream based on their positions,proper motions, and distances. The acquired sample permitted us to expand oursearch to nonvariable stars in the SDSS dataset, utilizing line-of-sightvelocities determined by the SDSS. We found 54 additional nonvariable starslinked to the Orphan stream. The metallicity distribution for the identifiedred giant branch stars and blue horizontal branch stars is, on average,$-2.13\pm0.05$ dex and $-1.87\pm0.14$ dex, with dispersions of 0.23 and0.43dex, respectively. The metallicity distribution of the RR~Lyrae variablespeaks at $-1.80\pm0.06$ dex and a dispersion of 0.25dex. Using the collectedstellar sample, we investigated a possible link between the ultra-faint dwarfgalaxy Grus II and the Orphan stream. Based on their kinematics, we found thatboth the stream RR~Lyrae and Grus II are on a prograde orbit with similarorbital properties, although the large uncertainties on the dynamicalproperties render an unambiguous claim of connection difficult. At the sametime, the chemical analysis strongly weakens the connection between both. Weargue that Grus II in combination with the Orphan stream would have to exhibita strong inverse metallicity gradient, which to date has not been detected inany Local Group system.

Milky Way archaeology using RR Lyrae and type II Cepheids I. The Orphan stream in 7D using RR Lyrae stars