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Using the Space Telescope Imaging Spectrograph, we have obtained ultraviolet(UV) spectra from $\sim 1200$ to 2000 \AA\ of known Lyman continuum (LyC)emitting galaxies at low redshift ($z \sim 0.3-0.4$) with varying absolute LyCescape fractions (fesc $\sim 0.01 - 0.72$). Our observations include inparticular the galaxy J1243+4646, which has the highest known LyC escapefraction at low redshift. While all galaxies are known Lyman alpha emitters, weconsistently detect an inventory of additional emission lines, including CIV1550, HeII 1640, OIII] 1666, and CIII] 1909, whose origin is presumablyessentially nebular. CIV 1550 emission is detected above 4 $\sigma$ in six outof eight galaxies, with equivalent widths of EW(CIV)$=12-15$ Ang for twogalaxies, which exceeds the previously reported maximum emission in low-$z$star-forming galaxies. We detect CIV 1550 emission in all LyC emitters withescape fractions fesc $> 0.1$ and find a tentative increase in the flux ratioCIV 1550/ CIII] 1909 with fesc. Based on the data, we propose a new criterionto select and classify strong leakers (galaxies with fesc $> 0.1$): CIV 1550/CIII] 1909 $> 0.75$. Finally, we also find HeII 1640 emission in all the strongleakers with equivalent widths from 3 to 8 Ang rest frame. These are among thehighest values observed in star-forming galaxies and are primarily due to ahigh rate of ionizing photon production. The nebular HeII 1640 emission of thestrong LyC emitters does not require harder ionizing spectra at $>54$ eVcompared to those of typical star-forming galaxies at similarly lowmetallicity.

Strong Lyman continuum emitting galaxies show intense CIV 1550 emission