EC 23257-5443: An Anomalous Reflection Effect Binary with Phase-Dependent Emission Features

EC 23257-5443 was classified as a hot subdwarf + F dwarf binary in Zones 4-6 of the Edinburgh-Cape Blue Object Survey nearly thirty years ago. Until recently, few observations of this relatively bright system have been published. High-cadence TESS photometry reveals a strong and coherent reflection effect variation with peak-to-peak amplitude of 50% and orbital period of 6.64 hr. The short orbital period draws into question the original sdB+F classification since such stars would fill their Roche lobes at this period. We obtained time-series spectroscopy with the Goodman spectrograph on the 4.1-m SOAR telescope and multi-color, time-series photometry with the CTIO 0.9-m telescope in order to shed further light on this system. Our spectroscopy reveals a spectrum that transitions back and forth between an absorption line spectrum and an emission line spectrum in phase with the photometric reflection effect. Our multi-color photometry shows that the reflection effect is significantly stronger at blue wavelengths than at red wavelengths - the opposite of what is typically seen in hot subdwarf reflection effect binaries. Here we present an analysis of our data and speculate on the true nature of EC 23257-5443.