On the gravitational lensing interpretation of three gravitational wave
  detections in the mass gap by LIGO and Virgo
  We search for gravitational wave (GW) events from LIGO-Virgo's third run that
may have been affected by gravitational lensing. Gravitational lensing delays
the arrival of GWs, and alters their amplitude -- thus biasing the inferred
progenitor masses. This would provide a physically well-understood
interpretation of GW detections in the "mass gap" between neutron stars and
black holes, as gravitationally lensed binary neutron star (BNS) mergers. We
selected three GW detections in LIGO-Virgo's third run for which the
probability of at least one of the constituent compact objects being in the
mass gap was reported as high with low latency -- i.e. candidate lensed BNS
mergers. Our observations of powerful strong lensing clusters located adjacent
to the peak of their sky localisation error maps reached a sensitivity $\rm
AB\simeq25.5$ in the $z'$-band with the GMOS instruments on the Gemini
telescopes, and detected no candidate lensed optical counterparts. We combine
recent kilonova lightcurve models with recent predictions of the lensed BNS
population and the properties of the objects that we followed up to show that
realistic optical counterparts were detectable in our observations. Further
detailed analysis of two of the candidates suggests that they are a plausible
pair of images of the same low-mass binary black hole merger, lensed by a local
galaxy or small group of galaxies. This further underlines that access to
accurate mass information with low latency would improve the efficiency of
candidate lensed NS-NS selection.
