The photometric period in ES Ceti

TL;DR

This study uses nine years of photometric data to analyze the orbital period evolution of the ultracompact binary ES Ceti, providing insights into its donor star and formation channel.

Contribution

It offers the first long-term period tracking of ES Ceti, supporting the double white dwarf formation scenario through observational evidence.

Findings

No significant period deviation from linear ephemeris

Donor star is likely small and near full degeneracy

Supports double white dwarf formation channel

Abstract

We present ULTRACAM photometry of ES Cet, an ultracompact binary with a 620s orbital period. The mass transfer in systems such as this one is thought to be driven by gravitational radiation, which causes the binary to evolve to longer periods since the semi-degenerate donor star expands in size as it loses mass. We supplement these ULTRACAM+WHT data with observations made with smaller telescopes around the world over a nine year baseline. All of the observations show variation on the orbital period, and by timing this variation we track the period evolution of this system. We do not detect any significant departure from a linear ephemeris, implying a donor star that is of small mass and close to a fully degenerate state. This finding favours the double white dwarf formation channel for this AM CVn star. An alternative explanation is that the system is in the relatively short-lived phase in which the mass transfer rate climbs towards its long-term value.