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SN2010da/NGC 300 ULX-1 was first detected as a supernova impostor in May 2010and was recently discovered to be a pulsating ultraluminous X-ray source. Inthis letter, we present VLT/X-shooter spectra of this source obtained inOctober 2018, covering the wavelength range 350-2300 nm. The $J$- and $H$-bandsclearly show the presence of a red supergiant donor star that is best matchedby a MARCS stellar atmosphere with $T_{\rm eff} = 3650 - 3900$ K and$\log(L_{\rm bol}/L_{\odot}) = 4.25\pm0.10$, which yields a stellar radius $R =310 \pm 70 R_{\odot}$. To fit the full spectrum, two additional components arerequired: a blue excess that can be fitted either by a hot blackbody (T$\gtrsim 20,000$ K) or a power law (spectral index $\alpha \approx 4$) and islikely due to X-ray emission reprocessed in the outer accretion disk or thedonor star; and a red excess that is well fitted by a blackbody with atemperature of $\sim 1100$ K, and is likely due to warm dust in the vicinity ofSN2010da. The presence of a red supergiant in this system implies an orbitalperiod of at least 0.8-2.1 years, assuming Roche lobe overflow. Given the largedonor-to-compact object mass ratio, orbital modulations of the radial velocityof the red supergiant are likely undetectable. However, the radial velocityamplitude of the neutron star is large enough (up to 40-60 km s$^{-1}$) topotentially be measured in the future, unless the system is viewed at a veryunfavorable inclination.

Discovery of a red supergiant donor star in SN2010da/NGC 300 ULX-1