  It is shown that, in the case of early B-type main-sequence stars, of the
three ratios N/C, C/O, and N/O which are regarded as indicators of stellar
evolution, the ratio N/O is more reliable since it seems to be insensitive to
overionization of the NII and OII ions. On the other hand, the N/C and C/O
ratios, which include carbon, may contain systematic errors for stars with Teff
> 18500 K because of neglected overionization of CII ions. The ratio N/O is
studied in the atmospheres of 46 early main-sequence B stars. These values of
N/O are examined as functions of the effective temperature, age, rotation
speed, and mass of the stars. Most early B-stars in the main sequence are found
to have [N / O] = 0 , which indicates that N/O varies little during the main
sequence stage, and this result is independent of the basic parameters listed
above. There are two explanations for the large number of stars with [N / O] =
0 : it is predicted theoretically that for an initial rotation velocity V0 <
100 km/s, N/O varies little toward the end of the main sequence stage ([N/O] <
0.2) and observations show that most early main-sequence B-stars do actually
have low initial rotation velocities V0. The few early main-sequence B-stars
with higher [N/O] = 0.4-0.8 correspond to models with rotation velocities V0 =
200-300 km/s. This conclusion is consistent with earlier data for stars with
the same masses in a later stage of evolution: the AFG-supergiant and bright
giant stage.
