tokens: optical copper lines are free of non-local thermodynamic equilibrium (NLTE) effects (e.g. Mishenina et al. 2002 ). The first indication that something could be wrong with this statement came from Shi et al. ( 2014 ), who noted that the important NLTE mechanism affecting the Cu i spectrum is the ultraviolet (UV) overionization. Later, Yan, Shi Zhao ( 2015 ) investigated copper abundance in 64 late-type intermediate metal-poor stars from Galactic disc and halo. They succeeded in showing that NLTE effects are important for this atom (the correction is about 0.17 dex for metallicity −1.5). The NLTE calculations enhanced the copper abundance obtained in the range of metal-poor stars, giving a flatter distribution of [Cu/H] versus [Fe/H] than was declared in previous works based on the local thermodynamic equilibrium (LTE) assumption. This conclusion was confirmed by Yan et al. ( 2016 ) who stated that NLTE effects are strong for copper in metal-poor stars. Until now, the most comprehensive study of the Galactic evolution of copper in the light of NLTE computation was performed by Andrievsky et al. ( 2018 ). They showed that for a sample of intermediate and extreme metal-poor stars ([Fe/H] from −4.2 to −1.4) the mean [Cu/Fe] value is about −0.22 dex. This is very different from the results of LTE analyses, which show significant underabundance at the early stages of Galactic evolution. Very recently, Roederer Barklem ( 2018 ) performed new work on copper abundance in late-type stars using UV Cu ii lines, which are supposedly free from the influence of the NLTE effect. The Hubble Space Telescope ( HST ) and ground-based spectra were used for this aim. They showed that the mean copper abundance in six warm metal-poor ([Fe/H] from −2.50 to −0.95) dwarfs derived from Cu i and Cu ii lines differs by 0.36 dex (Cu ii lines give higher abundance compared with Cu i lines). Generally, this difference agrees with derived NLTE corrections in metal-poor stars reported by Yan et al. ( 2015 ) and Andrievsky et al. ( 2018 ). In this paper, we examine NLTE copper abundance in a sample of the same six metal-poor stars derived from different Cu i lines (including the far-UV region) and Cu ii lines. We aim to obtain an independent estimate of the reliability of our NLTE corrections, as reported in Andrievsky et al. ( 2018 ). 2 SPECTROSCOPIC MATERIAL We employed UV stellar spectra that were secured with the help of the Space Telescope Imaging Spectrograph (STIS; Kimble et al. 1998 ; Woodgate et al. 1998 ) on board the HST . Observations were made using the E230H echelle grating, the 0.09 x 0.2 arcsec 2 slit and the NUV Multianode Microchannel Array detector.The resolved power was R = 114 000. The investigated Cu i and Cu ii lines are situated in the range 2024−2248 Å. We retrieved archive spectra from the Mikulski Archive for Space Telescopes (MAST). All spectra are centred at 2013 Å. The following lines fall in the spectral range: Cu i 2024 Å and Cu ii 2037, 2055,
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