tokens: excluded in this work due to their CMD locations. Figure 1. Locations of the stars in the 7 deg 2 APOGEE field of NGC 6553. The solid circle indicates cluster tidal radius ( R t ). Non-members outside 1.5 R t are the grey crosses. Stars inside 1.5 R t are colour-coded, according to the colour bar shown in Fig. 2 . Cluster members and non-members are labelled by filled circles and crosses, respectively. Figure 2. [Fe/H] versus radial velocity. Symbol meanings are the same as in Fig. 1 . Figure 3. Differential reddening-corrected CMD of NGC 6553 from PSF photometry of VVV imaging, supplemented with bright stars from 2MASS. The cluster fiducial sequence is indicated as a red solid line. The grey dots are stars within 1.5 R t (tidal radius), and the black dots are stars within the half-light radius. APOGEE targets inside 1.5 R t are colour-coded as in Fig. 1 . Cluster members and non-members determined in this paper are labelled by filled circles and crosses, respectively. See text for more details. From the coordinate information, these 10 cluster members have no overlap with the spectroscopic studies of NGC 6553 published after the year 2006, which include Alves-Brito et al. ( 2006 ), Zoccali et al. ( 2008 ), Gonzalez et al. ( 2011 ), Johnson et al. ( 2014 ), and Dias et al. ( 2015 ). Most of the APOGEE spectra for our sample have signal-to-noise ratio (SNR) higher than 80, except the second star in Table 1 . Each star in our sample has a single one hour visit, so the SNR is correlated with the stellar brightness. The SNR plays an important role in estimating the uncertainties of element abundances (Majewski et al. 2015 ). Three stars are labelled as ‘PERSIST_HIGH’ and six are labelled as ‘PERSIST_MED’. Persistence, where a significant fraction of accumulated charge is released over a long period of time, is particularly strong in one of the detectors (1.51–1.58 μm) used in the SDSS III/APOGEE survey (Nidever et al. 2015 ). However, by comparing the results from spectra containing persistence pixels versus those without, Holtzman et al. ( 2015 ) suggest that persistence does not dramatically impact the parameters in DR12 data. Table 1. Basic parameters for cluster members of NGC 6553. Notes . a Differential reddening corrected magnitude. b Measurement error. c PERSIST: spectrum has a significant number (>20 per cent) of pixels in the high (or medium) persistence region. d N_WARN: parameter value is within half grid spacing of the synthetic spectrum grid edge for nitrogen. SN_WARN: SNR70. From APOGEE measurements, our 10 cluster members show a mean RV of −0.14 ± 5.47 3 km s −1 . Our mean RV agrees with the other determinations: 4 ± 7.1 km s −1 (Cohen et al. 1999 ), 1.6 ± 6 km s −1 (Meléndez et al. 2003 ), −1.86 ± 2.01 km s −1 (Alves-Brito et al. 2006 ), −2.03 ± 4.85 km s −1 (Johnson et al. 2014 ), and 6 ± 8 km s −1 (Dias et al. 2015 ). In addition, the APOGEE data yield a mean [Fe/H] of −0.15 ± 0.05. This GC has diverse metallicity results from
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