  We present high resolution observations of the giant extragalactic H II
regions NGC 604, NGC 2363, NGC 5461 and NGC 5471, based on observations taken
with the ISIS spectrograph on the William Herschel Telescope. We have detected
-by the first time- C II and O II recombination lines in these objects. We find
that recombination lines give larger C^{++} and O^{++} abundances than
collisionallly excited lines, suggesting that temperature variations can be
present in the objects. We detect [Fe IV] lines in NGC 2363 and NGC 5471, the
most confident detection of optical lines of this kind in H II regions.
Considering the temperature structure we derive their H, He, C, N, O, Ne, S,
Ar, and Fe abundances. From the recombination lines of NGC 5461 and NGC 5471 we
determine the presence of C/H and O/H gradients in M101. We calculate the Delta
Y/Delta O and Delta Y/Delta Z values considering the presence of temperature
variations and under the assumption of constant temperature. We obtain a better
agreement with models of galactic chemical evolution by considering the
presence of temperature variations than by assuming that the temperature is
constant in these nebulae.
