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The symbiotic X-ray binary Sct X-1 was suggested as the first known neutronstar accreting from a red supergiant companion. Although known for nearly 50years, detailed characterization of the donor remains lacking, particularly dueto the extremely high reddening towards the source ($A_V\gtrsim25$ mag). Here,we present i) improved localization of the counterpart using Gaia and Chandraobservations, ii) the first broadband infrared spectrum ($\approx1-5\,\mu$m;$R\approx 2000$) obtained with SpeX on the NASA Infrared Telescope Facility andiii) $J$-band light curve from the Palomar Gattini-IR survey. The infraredspectrum is characterized by i) deep water absorption features (H$_2$O index$\approx 40$%), ii) strong TiO, VO and CO features, and iii) weak/absent CNlines. We show that these features are inconsistent with known red supergiants,but suggest a M8-9 III type O-rich Mira donor star. We report the discovery oflarge amplitude ($\Delta J\approx3.5$ mag) periodic photometric variabilitysuggesting a pulsation period of$621\pm36\,{\rm(systematic)}\pm8\,{\rm(statistical)}$ days, which we use toconstrain the donor to be a relatively luminous Mira ($M_K=-8.6\pm0.3$ mag) ata distance of $3.6^{+0.8}_{-0.7}$ kpc. Comparing these characteristics torecent models, we find the donor to be consistent with a $\approx 3-5$M$_\odot$ star at an age of $\approx 0.1-0.3$ Gyr. Together, we show that SctX-1 was previously mis-classified as an evolved High Mass X-ray Binary; insteadit is an intermediate mass system with the first confirmed Mira donor in anX-ray binary. We discuss the implications of Mira donors in symbiotic X-raybinaries, and highlight the potential of wide-field infrared time domainsurveys and broadband infrared spectroscopy to unveil their demographics.

A massive AGB donor in Scutum X-1: Identification of the first Mira variable in an X-ray binary