  A long-lived $J^{\pi}=4_1^+$ isomer, $T_{1/2}=2.2(1)$ms, has been discovered
at 643.4(1) keV in the weakly-bound $^{26}_{9}$F nucleus. It was populated at
GANIL in the fragmentation of a $^{36}$S beam. It decays by an internal
transition to the $J^{\pi}=1_1^+$ ground state (82(14)%), by $\beta$-decay to
$^{26}$Ne, or beta-delayed neutron emission to $^{25}$Ne. From the beta-decay
studies of the $J^{\pi}=1_1^+$ and $J^{\pi}=4_1^+$ states, new excited states
have been discovered in $^{25,26}$Ne. Gathering the measured binding energies
of the $J^{\pi}=1_1^+-4_1^+$ multiplet in $^{26}_{9}$F, we find that the
proton-neutron $\pi 0d_{5/2} \nu 0d_{3/2}$ effective force used in shell-model
calculations should be reduced to properly account for the weak binding of
$^{26}_{9}$F. Microscopic coupled cluster theory calculations using
interactions derived from chiral effective field theory are in very good
agreement with the energy of the low-lying $1_1^+,2_1^+,4_1^+$ states in
$^{26}$F. Including three-body forces and coupling to the continuum effects
improve the agreement between experiment and theory as compared to the use of
two-body forces only.
