The ON stars are a rare subtype of O stars , of uncertain origin .
We report two new , rapidly-rotating ON stars found in data acquired with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope , LAMOST .
LS I +61 28 is an ON8.5 Vn dwarf with a projected equatorial rotational velocity of \mbox { { $v _ { \textrm { e } } \sin { i } $ } } \simeq 298 km s ^ { -1 } , while HDE 236672 is an ON9 IVn subgiant with \mbox { { $v _ { \textrm { e } } \sin { i } $ } } \simeq 253 km s ^ { -1 } .
The former is the first rapidly-rotating ON dwarf to be found , and the latter is only the third ON subgiant .
The luminosity classes of non-supergiant ON stars appear to be influenced by the axial inclination angle i : the rapidly-rotating giants are close to equator-on , while ON dwarfs with lower v _ { \textrm { e } } \sin { i } values are viewed more nearly pole-on .
Combining parallaxes and proper motions from Gaia DR2 with radial-velocity measurements , we investigate the kinematics of non-supergiant ON stars , and infer that the dynamics , rapid rotation , and surface-nitrogen characteristics may all be consequences of binary interaction .