  Non-LTE abundances of magnesium, aluminum and sulfur are derived for a sample
of 23 low-v \sin i stars belonging to six northern OB associations of the
Galactic disk within 1 kpc of the Sun. The abundances are obtained from the
fitting of synthetic line profiles to high resolution spectra. A comparison of
our results with HII region abundances indicates good agreement for sulfur
while the cepheid abundances are higher. The derived abundances of Mg show good
overlap with the cepheid results. The aluminum abundances for OB stars are
significantly below the cepheid values. But, the OB star results show a
dependence with effective temperature and need further investigation. The high
Al abundances in the cepheids could be the result of mixing. A discussion of
the oxygen abundance in objects near the solar circle suggests that the current
mean galactic oxygen abundance in this region is 8.6-8.7 and in agreement with
the recently revised oxygen abundance in the solar photosphere. Meaningful
comparisons of the absolute S, Al and Mg abundances in OB stars with the Sun
must await a reinvestigation of these elements, as well as the meteoritic
reference element Si, with 3D hydrodynamical model atmospheres for the Sun. No
abundance gradients are found within the limited range in galactocentric
distances in the present study. Such variations would be expected only if there
were large metallicity gradients in the disk.
