  GW170817 showed that neutron star mergers not only emit gravitational waves
but also can release electromagnetic signatures in multiple wavelengths. Within
the first half of the third observing run of the Advanced LIGO and Virgo
detectors, there have been a number of gravitational wave candidates of compact
binary systems for which at least one component is potentially a neutron star.
In this article, we look at the candidates S190425z, S190426c, S190510g,
S190901ap, and S190910h, predicted to have potentially a non-zero remnant mass,
in more detail. All these triggers have been followed up with extensive
campaigns by the astronomical community doing electromagnetic searches for
their optical counterparts; however, according to the released classification,
there is a high probability that some of these events might not be of
extraterrestrial origin. Assuming that the triggers are caused by a compact
binary coalescence and that the individual source locations have been covered
during the EM follow-up campaigns, we employ three different kilonova models
and apply them to derive possible constraints on the matter ejection consistent
with the publicly available gravitational-wave trigger information and the lack
of a kilonova detection. These upper bounds on the ejecta mass can be related
to limits on the maximum mass of the binary neutron star candidate S190425z and
to constraints on the mass-ratio, spin, and NS compactness for the potential
black hole-neutron star candidate S190426c. Our results show that deeper
electromagnetic observations for future gravitational wave events near the
horizon limit of the advanced detectors are essential.
