  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 LyC
escape fractions (fesc $\sim 0.01 - 0.72$). Our observations include in
particular the galaxy J1243+4646, which has the highest known LyC escape
fraction at low redshift. While all galaxies are known Lyman alpha emitters, we
consistently detect an inventory of additional emission lines, including CIV
1550, HeII 1640, OIII] 1666, and CIII] 1909, whose origin is presumably
essentially nebular. CIV 1550 emission is detected above 4 $\sigma$ in six out
of eight galaxies, with equivalent widths of EW(CIV)$=12-15$ Ang for two
galaxies, which exceeds the previously reported maximum emission in low-$z$
star-forming galaxies. We detect CIV 1550 emission in all LyC emitters with
escape fractions fesc $> 0.1$ and find a tentative increase in the flux ratio
CIV 1550/ CIII] 1909 with fesc. Based on the data, we propose a new criterion
to 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 strong
leakers with equivalent widths from 3 to 8 Ang rest frame. These are among the
highest values observed in star-forming galaxies and are primarily due to a
high rate of ionizing photon production. The nebular HeII 1640 emission of the
strong LyC emitters does not require harder ionizing spectra at $>54$ eV
compared to those of typical star-forming galaxies at similarly low
metallicity.
