  Superluminous supernovae (SLSNe) are at least $\sim$5 times more luminous
than common supernovae (SNe). Especially hydrogen-poor SLSN-I are difficult to
explain with conventional powering mechanisms. One possible scenario that might
explain such luminosities is that SLSNe-I are powered by an internal engine,
such as a magnetar or an accreting black hole. Strong magnetic fields or
collimated jets can circularly polarize light. In this work, we measured
circular polarization of two SLSNe-I with the FOcal Reducer and low dispersion
Spectrograph (FORS2) mounted at the ESO's Very Large Telescope (VLT). PS17bek,
a fast evolving SLSN-I, was observed around peak, while OGLE16dmu, a slowly
evolving SLSN-I, was observed 100 days after maximum. Neither SLSN shows
evidence of circularly polarized light, however, these non-detections do not
rule out the magnetar scenario as the powering engine for SLSNe-I. We calculate
the strength of the magnetic field and the expected circular polarization as a
function of distance from the magnetar, which decreases very fast.
Additionally, we observed no significant linear polarization for PS17bek at
four epochs, suggesting that the photosphere near peak is close to spherical
symmetry.
