  Radioactive beams of $^{14}$O and $^{15}$O were used to populate the resonant
states 1/2$^+$, 5/2$^+$ and $0^-,1^-,2^-$ in the unbound $^{15}$F and $^{16}$F
nuclei respectively by means of proton elastic scattering reactions in inverse
kinematics. Based on their large proton spectroscopic factor values, the
resonant states in $^{16}$F can be viewed as a core of $^{14}$O plus a proton
in the 2s$_{1/2}$ or 1d$_{5/2}$ shell and a neutron in 1p$_{1/2}$. Experimental
energies were used to derive the strength of the 2s$_{1/2}$-1p$_{1/2}$ and
1d$_{5/2}$-1p$_{1/2}$ proton-neutron interactions. It is found that the former
changes by 40% compared with the mirror nucleus $^{16}$N, and the second by
10%. This apparent symmetry breaking of the nuclear force between mirror nuclei
finds explanation in the role of the large coupling to the continuum for the
states built on an $\ell=0$ proton configuration.
