tokens: 4026, Si iv 4089, He i 4144, He ii 4200, He i 4387, He i 4471, He ii 4542, Si iii 4552, He ii 4686 and He i 4713. From the observed ratio values, we used the tabulated and graphical results of Martins ( 2018 ) to define spectral types and luminosity classes, as well as, if necessary, the associated qualifiers. When most of the measured ratios indicate the same result, the associated spectral type is then assigned to the star. However, when a satisfactory agreement is not obtained, the distinct types are then used as initial estimates, and a better tuning on the spectral type of the star is then obtained from the comparison of the observed spectrum with those of standards. Figure 1. Normalized blue-optical spectra (4000–4950 Å) of the newly identified O-stars in the LAMOST DR3. The main spectral features used in the blue-optical canonical spectral classification process are labelled. LS V +39 27 (O4 V), together with J20261845+4347472 (O5 III) and TYC 3339-851-1 (O5 V) are the earliest exemplars in our sample, which is complemented by several mid- to late-type O-stars. Figure 2. Normalized blue-optical spectra (4000–4950 Å) of the newly identified O-stars in the LAMOST DR3. The main spectral features used in the blue-optical canonical spectral classification process are labelled. LS V +39 27 (O4 V), together with J20261845+4347472 (O4 IV(f)) and TYC 3339-851-1 (O5 v ((f))) are the earliest exemplars in our sample, which is complemented by several mid- to late-type O-stars. 3.1 Spectral types and luminosity classes In order to obtain the EWs and full width at half-maximum (FWHM) of the spectral lines necessary for the spectral typing process, we followed standard procedures briefly described here. The line-profile parameters were measured using several spectroscopic analysis routines available in the iraf 2 splot package. For each spectral line, we performed model fittings using both Voigt and pure Gaussian profiles, with the associated continuum computed from regions far from the observed line wings (in general, several tens of angstroms from the estimated line centres), with the final uncertainty in the EW and FWHM values ranging from about 10 per cent for spectra with the highest S/N (usually above 300–350) to 15–20 per cent for the spectra presenting lower S/N values. Table 2 presents the EW values measured from all spectra of stars in Table 1 , for the following absorption lines: (i) eight He lines corresponding to the He i-ii λ4026, He i λ4121, He ii λ4200, He i λ4387, He i λ4471, He ii λ4542, He ii λ4686 and He ii λ4713 transitions; (ii) two Si lines corresponding (when present) to the Si iv λ4089 and Si iii λ4552 transitions. Table 2. Equivalent widths (Å) and associated errors of selected transition lines measured in the LAMOST DR3 O-star spectra. From inspection of Figs 1 and 2 , and from the measured parameters for the He ii lines, we can see that the earliest O-stars in our sample are stars 10, 24, 3, 8, 14 and 19. By following the
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