  A search is presented for fractionally charged particles with charge below
1$e$, using their small energy loss in the tracking detector as a key variable
to observe a signal. The analyzed data set corresponds to an integrated
luminosity of 138 fb$^{-1}$ of proton-proton collisions collected at $\sqrt{s}$
= 13 TeV in 2016-2018 at the CERN LHC. This is the first search at the LHC for
new particles with charges between $e/$3 and $e$. Masses up to 640 GeV and
charges as low as $e/$3 are excluded at 95% confidence level. These are the
most stringent limits to date for the considered Drell-Yan-like production
mode.
