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tokens: that all six galaxies are LyC leakers with f esc (LyC) ranging between 6 and 46 per cent. These values are much higher than f esc (LyC) of ∼0–4.5 per cent derived e.g. by Borthakur et al. ( 2014 ) and Chisholm et al. ( 2017 ) in SFGs with lower O 32 or with lower equivalent widths of the H β emission line (Hernandez et al. 2018 ). In this paper we present new HST /COS observations of the LyC in five compact SFGs with the highest O 32 ∼ 8–27 ever observed in order to detect ionizing radiation, and examine its behaviour over a wide range of O 32 . The properties of selected SFGs derived from observations in the optical range are presented in Section 2 . The HST observations and data reduction are described in Section 3 . The surface brightness (SB) profiles in the UV range are discussed in Section 4 . In Section 5 we compare the HST /COS spectra with the extrapolation of the modelled SEDs to the UV range. Lyα emission is considered in Section 6 . The escaping LyC emission is discussed in Section 7 together with the corresponding escape fractions. In Section 8 our results are compared with the LyC escape fractions for other galaxies obtained in some recent studies. We summarize our findings in Section 9 . 2 PROPERTIES OF SELECTED GALAXIES DERIVED FROM OBSERVATIONS IN THE OPTICAL RANGE 2.1 Emission-line diagnostic diagrams Our LyC leaker candidates were selected from the SDSS Data Release 12 (DR12) (Alam et al. 2015 ) by adopting the selection criteria described in Izotov et al. ( 2016a,c , 2018 ). All of them have a compact structure on SDSS images (Fig. 1 ) and high equivalent widths EW(H β) > 200 Å of the H β emission line in the SDSS spectra, indicating very recent star formation. These galaxies are located in the upper part of the SFG branch in the Baldwin–Phillips–Terlevich (BPT) diagram (filled stars in Fig. 2a ) implying the presence of high-excitation H ii regions. Izotov et al. ( 2016a,c ) discussed only galaxies with O 32 ∼ 5–7. In this paper and in Izotov et al. ( 2018 ) we report observations of galaxies with the considerably larger range of O 32 ∼ 8–27 (Table 1 ), aiming to study the applicability of the O 32 criterion to select galaxies with high escape fractions of ionizing radiation. It is seen in Fig. 2(b) that the selected galaxies have the highest O 32 ratios among all local LyC leaking galaxies observed so far. Figure 1. 25 arcsec × 25 arcsec SDSS composite images of selected galaxies. Figure 2. (a) The BPT diagram (Baldwin, Phillips Terlevich 1981 ) for SFGs. (b) The O 32 –R 23 diagram for SFGs where R 23 = ([O ii ]3727 + [O iii ]4959 + [O iii ]5007)/H β. The LyC leaking galaxies (this paper) and J1154+2443 (Izotov et al. 2018 ) are shown by filled stars in both panels. The location of LyC leaking galaxies by Borthakur et al. ( 2014 ) and Chisholm et al. ( 2017 ) are shown by open circles while LyC leaking galaxies by Izotov et al. ( 2016a,c ) are represented by open stars. The compact SFGs (Izotov et al. 2016b ) are