2025RNAAS...9..161C

We report the discovery of an SX Phoenicis-type pulsating variable star via 217 epochs of time-series photometry from the Vera C. Rubin Observatory's Data Preview 1. The star, designated LSST-DP1-O-614435753623041404 (or LSST-C25_var1 for short), has mean magnitudes of (〈g〉, 〈r〉) = (18.65, 18.63), with pulsation amplitudes of (0.60, 0.38) mag in these bands. Its period is 0.0767day (1.841 hr), typical of SX Phe pulsators. We derive a distance to the star of 16.6 kpc based on an SX Phe period-luminosity relation. Its position ∼5 kpc from the Galactic plane, in the outer Milky Way disk at a Galactocentric distance of ∼22 kpc, and its proper motion suggest that LSST-C25_var1 is part of the Monoceros Ring structure. This star is presented as a small taste of the many thousands of variable stars expected in Rubin/Legacy Survey of Space and Time data.