An intermediate-mass stripped star with a dark companion in the Small Magellanic Cloud

We present discovery of SMASH J004005$-$733351, an intermediate-mass stripped helium star in the Small Magellanic Cloud with a dark companion. The system was identified in a new optical color-selected survey for stripped stars and confirmed through spectroscopic follow-up. We measure an orbital period of $P_{\rm orb}=19.95\pm 0.002$ hours and a radial-velocity semi-amplitude of $K=91.7 ^{+3.2}_{-3.1}\,{\rm km\,s^{-1}}$. Simultaneous modeling of the optical spectrum and ultraviolet-to-optical spectral energy distribution yields a stripped star evolutionary mass $M_{\rm stripped} = 3.84 \pm 0.54\,M_\odot$, temperature $T_{\rm eff} = 68.3 \pm 6.4$ kK, surface gravity $\log g= 4.76 \pm 0.12$, luminosity $\log(L/L_\odot)= 4.28 \pm 0.13$, radius $R= 0.99 \pm 0.09\,R_\odot$, and an unusually high surface hydrogen abundance of $X_{\rm H, s}=0.63 \pm 0.04$. The companion is undetected spectroscopically, and the orbital solution and stripped-star evolutionary mass imply a minimum companion mass of $M_2 \gtrsim 1.2\,M_\odot$. The absence of photometric variability from irradiation rules out a main-sequence companion. The companion is most likely a neutron star (NS), although a massive white dwarf or low-mass stripped star cannot be fully excluded. Binary evolution models predict that the stripped star will refill its Roche lobe after core helium exhaustion, likely producing a stripped or ultra-stripped supernova and ultimately a double NS binary if the system survives. SMASH J004005$-$733351 is among the first known stripped stars with a compact object companion. The system provides a new empirical benchmark for the late stages of binary interaction that lead to (ultra-)stripped supernovae and double NS binaries.