  We present observations with the Cosmic Origins Spectrograph onboard the
Hubble Space Telescope of seven compact low-mass star-forming galaxies at
redshifts, z,in the range 0.3161-0.4276, with various O3Mg2=[OIII]5007/MgII
2796+2803 and Mg2=MgII 2796/MgII 2803 emission-line ratios. We aim to study the
dependence of leaking Lyman continuum (LyC) emission on the characteristics of
MgII emission together with the dependences on other indirect indicators of
escaping ionizing radiation. LyC emission with escape fractions
fesc(LyC)=3.1-4.6 per cent is detected in four galaxies, whereas only 1sigma
upper limits of fesc(LyC) in the remaining three galaxies were derived. A
strong narrow Ly-alpha emission line with two peaks separated by Vsep~298-592
km/s was observed in four galaxies with detected LyC emission and very weak
Ly-alpha emission is observed in galaxies with LyC non-detections. Our new data
confirm the tight anti-correlation between fesc(LyC) and Vsep found for
previous low-redshift galaxy samples. Vsep remains the best indirect indicator
of LyC leakage among all considered indicators. It is found that escaping LyC
emission is detected predominantly in galaxies with Mg2>1.3. A tendency of an
increase of fesc(LyC) with increasing of both the O3Mg2 and Mg2 is possibly
present. However, there is substantial scatter in these relations not allowing
their use for reliable prediction of fesc(LyC).
