Discovery of an SX Phe Star in the Euclid Deep Field South from Rubin Observatory LSST Alerts

We report the discovery of an SX Phe-type pulsating star LSST-AP-DO-313893022856118297 from the Rubin Observatory Legacy Survey of Space and Time (LSST) alerts, which is located within one of the Deep Drilling Fields (DDFs), the Euclid Deep Field South. From LSST and our own observations, its pulsation period is determined as 0.04638(1) day. The Southern African Large Telescope optical spectrum with the deep absorption lines of the Balmer series is well-fitted by a stellar atmosphere model with effective temperature T eff = 7650 ± 200 K, surface gravity log g ( cm s − 2 ) = 4.6 ± 0.2 , and metallicity [Fe/H] = −1.4 ± 0.2. Together with its high Galactic latitude ( b = − 49 . ° 3535 ), these characteristics are consistent with the general properties of SX Phe stars. This result emphasizes the importance of the DDF even for Galactic science cases.