  Binary mass transfer can occur at high rates due to rapid expansion of the
donor's envelope. In the case where mass transfer is unstable, the binary can
rapidly shrink its orbit and lead to a merger. In this work we consider the
appearance of the system preceding merger, specifically for the case of a
low-mass ($\approx 2.5$-$3~M_\odot$) helium star with a neutron star (NS)
companion. Modeling the mass transfer history as well as the wind launched by
super-Eddington accretion onto the NS, we find that such systems can power
slowly rising transients with timescales as long as years, and luminosities of
$\sim 10^{40}$-$10^{41}$ erg s$^{-1}$ from optical to UV. The final explosion
following the merger (or core-collapse of the helium star in some cases) leads
to an interaction-powered transient with properties resembling Type Ibn
supernovae (SNe), possibly with a bright early peak powered by shock cooling
emission for merger-powered explosions. We apply our model to the Type Ibn SN
2023fyq, that displayed a long-term precursor activity from years before the
terminal explosion.
