  We present measurements of the gas-phase C/O abundance ratio in six H II
regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet
spectroscopy using the Faint Object Spectrograph on the Hubble Space Telescope.
The C/O ratios increase systematically with O/H in both galaxies, from log C/O
approximately -0.8 at log O/H = -4.0 to log C/O approx. -0.1 at log O/H = -3.4.
C/N shows no correlation with O/H. The rate of increase of C/O is somewhat
uncertain because of uncertainty as to the appropriate UV reddening law, and
uncertainty in the metallicity dependence on grain depletions. However, the
trend of increasing C/O with O/H is clear, confirming and extending the trend
in C/O indicated previously from observations of irregular galaxies. Our data
indicate that the radial gradients in C/H across spiral galaxies are steeper
than the gradients in O/H. Comparing the data to chemical evolution models for
spiral galaxies shows that models in which the massive star yields do not vary
with metallicity predict radial C/O gradients that are much flatter than the
observed gradients. The most likely hypothesis at present is that stellar winds
in massive stars have an important effect on the yields and thus on the
evolution of carbon and oxygen abundances. C/O and N/O abundance ratios in the
outer disks of spirals determined to date are very similar to those in dwarf
irregular galaxies. This implies that the outer disks of spirals have average
stellar population ages much younger than the inner disks.
