tokens: a slit orientation to better separate the components of the counterpart, but the seeing was not good enough to resolve them. In these observations, spectra of three objects are visible. Source 2 (see Fig. 1 ) corresponds to the elongated counterpart. All three spectra (sources 1 to 3) show a continuum with many absorption lines from neutral metals. The CO bandheads at 1.62 and 1.66 μm are also detected, proving that the objects are all K- or M-type stars (see Fig. 2 for the spectrum of source 2). We find no evidence for velocity broadening of the absorption lines. This is fully consistent with what we expect for RSGs, which have rotational velocities of at most a few km s −1 , undetectable at MOSFIRE's resolution of ∼80 km s −1 in the H band. Figure 2. The normalized spectra of NGC 925 J022721+333500 (source 2, from the Dec. 2013/Jan. 2014 observation, blue line) and a 3500 K model from Lançon et al. ( 2007 , red line, shifted downwards by 0.5 for clarity). The spectrum of J022721+333500 has been shifted by −506 km s −1 and has been interpolated over wavelength ranges that were strongly affected by noise from telluric emission lines (grey shaded areas). The ‘emission lines’ visible near some of these ranges are due to this noise. The dashed lines indicate the positions of absorption lines that are indicative of late-type stars – from short to long wavelength: Mg i triplet (1.574, 1.575, 1.577 μm), Si i (1.59 μm), CO bandhead (1.62 μm), CO bandhead (1.66 μm), Si i (1.67 μm), Al i triplet (1.672, 1.675, 1.676 μm), OH (1.69 μm) and Mg i (1.71 μm). From the cross-correlation with template spectra, we find a radial velocity of 495 ± 11 km s −1 for source 1. For source 2, we find a radial velocity of 506 ± 9 km s −1 for the Dec. 2013/Jan. 2014 spectrum and 500 ± 14 km s −1 for the Nov 2014 spectrum. For source 3, we find a radial velocity of 502 ± 8 km s −1 . This source is not centred on the slit, which causes an offset in the radial velocity. The sources are too faint to be visible in our through-slit alignment images. However, brighter stars selected to check the slit alignment were detected and we use the comparison of their locations on the through-slit image with that of the deep WHT/LIRIS image (Heida et al. 2014 ) to determine the actual location of the slit on the sky. From this, we derive a best estimate of the offset between the location of source 3 and the centre of the slit of 0.5 arcsec ± 0.05 arcsec (see Fig. 1 ). This corresponds to 2.8 ± 0.3 pixels on the MOSFIRE detector, or a velocity offset of 85 ± 10 km s −1 . Because the seeing was comparable to the slit width, this value is an upper limit on the velocity offset. If we correct for this, we find a radial velocity ≥420 ± 15 km s −1 for source 3. The radial velocities of sources 1 and 2 are in agreement with what is expected from the radial velocity field of NGC 925 (de Blok et al. 2008 ). This proves that the objects are located in NGC 925, confirming the absolute magnitude
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