  We present observations with the Space Telescope Imaging Spectrograph (STIS)
onboard the Hubble Space Telescope of eleven Lyman continuum (LyC) leaking
galaxies at redshifts, z, in the range 0.29-0.43, with oxygen abundances
12+log(O/H)=7.64-8.16, stellar masses Mstar~10^7.8-10^9.8 Msun and
O32=[OIII]5007/[OII]3727 of ~5-20, aiming to detect CIII]1908 emission line. We
combine these observations with the optical Sloan Digital Sky Survey (SDSS)
spectra for the determination of carbon, nitrogen and oxygen abundances. Our
sample was supplemented by thirty one galaxies from the literature, for which
carbon, nitrogen and oxygen abundances can be derived from the HST and SDSS
spectra. These additional galaxies, however, do not have LyC observations. We
find that log(C/O) for the entire sample at 12+log(O/H)<8.1 does not depend on
metallicity, with a small dispersion of ~0.13 dex around the average value of ~
-0.75 dex. On the other hand, the log(N/O) in galaxies at z>0.1, including LyC
leakers, is systematically higher compared to the rest of the sample with lower
metallicity. We find that log(C/O) slightly decreases with increasing Mstar
from ~ -0.65 at Mstar=10^6 Msun to ~ -0.80 at Mstar=10^9-10^10 Msun, whereas
log(N/O) is considerably enhanced at Mstar>10^8 Msun. The origin of these
trends remains basically unknown. One of the possible solutions is to assume
that the upper mass limit of the stellar initial mass function (IMF) in more
massive galaxies is higher. This would result in higher production of oxygen
and larger fraction of massive stars with stellar wind polluting interstellar
medium with nitrogen.
