  Most stars are born in stellar clusters and their protoplanetary disks, which
are the birthplaces of planets, can therefore be affected by the radiation of
nearby massive stars. However, little is known about the chemistry of
externally irradiated disks, including whether or not their properties are
similar to the so-far better-studied isolated disks. Motivated by this
question, we present ALMA Band 6 observations of two irradiated Class II
protoplanetary disks in the outskirts of the Orion Nebula Cluster (ONC) to
explore the chemical composition of disks exposed to (external) FUV radiation
fields: the 216-0939 disk and the binary system 253-1536A/B, which are exposed
to radiation fields of $10^2-10^3$ times the average interstellar radiation
field. We detect lines from CO isotopologues, HCN, H$_2$CO, and C$_2$H toward
both protoplanetary disks. Based on the observed disk-integrated line fluxes
and flux ratios, we do not find significant differences between isolated and
irradiated disks. The observed differences seem to be more closely related to
the different stellar masses than to the external radiation field. This
suggests that these disks are far enough away from the massive Trapezium stars,
that their chemistry is no longer affected by external FUV radiation.
Additional observations towards lower-mass disks and disks closer to the
massive Trapezium stars are required to elucidate the level of external
radiation required to make an impact on the chemistry of planet formation in
different kinds of disks.
