  We present observations with the Cosmic Origins Spectrograph onboard the
Hubble Space Telescope of five star-forming galaxies at redshifts z in the
range 0.2993-0.4317 and with high emission-line flux ratios
O32=[OIII]5007/[OII]3727 ~ 8-27 aiming to detect the Lyman continuum (LyC)
emission. We detect LyC emission in all galaxies with the escape fractions
fesc(LyC) in a range of 2-72 per cent. A narrow Ly-alpha emission line with two
peaks in four galaxies and with three peaks in one object is seen in
medium-resolution COS spectra with a velocity separation between the peaks Vsep
varying from ~153 km/s to ~345 km/s. We find a general increase of the LyC
escape fraction with increasing O32 and decreasing stellar mass M*, but with a
large scatter of fesc(LyC). A tight anti-correlation is found between fesc(LyC)
and Vsep making Vsep a good parameter for the indirect determination of the LyC
escape fraction. We argue that one possible source driving the escape of
ionizing radiation is stellar winds and radiation from hot massive stars.
