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tokens: 5 5 TYC 1323-1592-1 (=LS 36; Stephenson Sanduleak 1971 ) is an O8 Vz star newly identified here. It has the narrowest line profiles among the LAMOST medium-resolution spectra of O dwarfs, hence, we assume ≃ 0; the exact value is of little consequence, as long as it is small (which is clearly the case). to estimate P ( λ ), which is the rectified intrinsic line profile without rotation. 2. For an assumed , we fitted the functionto S ( λ ), which is the observed He i λ 5016 profile for each ON star. Here, ⨂ is the convolution operator; is the rotational broadening function (Gray 2005 ); and are fit parameters. 3. A figure of merit, , was calculated for trial values from 180 to 400 at steps of 7 . A refined best-fit was finally obtained by fitting a third-order polynomial to the 11 ξ values about the smallest one for each star. To estimate (internal) uncertainties on the velocities, we assumed the best-fit model to be representative of the true profile, added Gaussian noise matching the observed spectra, and measured v R and v e sin i as above, repeating the process 10 4 times. Our final results for and are: (LS I +61 28) and (HDE 236672). The He i λ 5016 rectified observations and adopted models are shown in Figure 2 . If macroturbulence were more important in the target stars than in TYC 1323-1592-1, our values may be upper limits (Howarth et al. 1997 ; Simón-Díaz Herrero 2007 ), although rotational broadening clearly dominates for the rapid rotators. Figure 2. He i λ 5016 profiles of LS I +61 28 and HDE 236672. The red lines are the best-fit rotationally broadened models. We also measured the radial velocity of He i λ 5016, along with other lines of hydrogen and helium, in the low-resolution spectra at our disposal, and find no evidence for radial-velocity variability in excess of ∼20 (i.e., ∼0.1 of the resolution element). 3. Kinematics of Galactic ON Stars Gaia DR2 provides parallaxes and proper motions with unprecedented precision for more than a billion stars (Gaia Collaboration et al. 2018 ). By combining this information with radial velocities, we can determine the kinematics of ON stars in the Galaxy. In addition to the two new ONn stars reported here, we investigate all those ON stars satisfying the following criteria: (i) not classified as a supergiant, (ii) parallax accuracy of , and (iii) a radial velocity reported in Table 2 of Walborn et al. ( 2011 ). This sample is summarized in Table 2 . Other than for their rotation velocities, the dozen sample stars have similar spectral characteristics and similar effective temperatures (and thus may have arisen through similar processes). Table 2 Kinematic Parameters for the ON-star Sample Note. Spectral types (column 3) and binary indicators (column 10) adopted from Walborn et al. ( 2011 ), except HDE 236672 and LS I +61 28 (this paper). Projected rotational velocities are from Martins et al. ( 2015b ), Howarth et al. ( 1997 ), and this paper. In column 9, . We used Markov Chain Monte