An eclipsing CEMP candidate discovered in a search for dwarf carbon stars in post-common envelope binaries

Abstract

Dwarf carbon stars are dominated by members of the Galactic halo and are thus likely carbon-enhanced metal-poor stars. In this work, a sample of 879 bona fide dwarf carbon stars are characterized by their ground-based light curves, and p<15 d modulation is found to be significant in 31 objects (3.5%), consistent with starspots and rotation in tidally-locked, post-common envelope binaries. Among these is an unambiguous halo star that is eclipsing every 1.224 d, and where the 30% eclipse depth rules out a white dwarf occulter. Available Gaia data do not indicate any tertiary in the eclipsing system, but this remains a possibility and follow-up data are necessary to determine the evolutionary history of this first eclipsing binary among carbon-enhanced stars. Four of the variable sources exhibit clear multi-year, quasi-sinusoidal trends indicative of magnetic-activity and starspot cycles in rapidly-rotating, dynamo-rejuvenated stars. These data support a picture where carbon pollution results from wind capture prior to Roche lobe overflow, and the orbital period distribution appears to be moderately shifted to longer periods than carbon-normal, low-mass stars in similar binaries. The band-combined approach adopted in this work may be more sensitive than prior work using single-bandpass light curves, where at most 19 of 34 binary candidates published by Roulston et al. (2021) are independently confirmed here.