The vanishing of the long photometric cycle in AU Monocerotis

¹ Instituto de Física y Astronomía, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
² Departamento de Astronomía, Universidad de Concepción, Av. Esteban Iturra s/n Casilla 160-C, Concepción, Chile
³ Centro de Investigación en Ciencias del Espacio y Física Teórica, Universidad Central de Chile, Av. Francisco de Aguirre 0405, La Serena, Chile
⁴ Astronomical Observatory, Volgina 7, 11000 Belgrade, Serbia

^⋆ Corresponding author: lientur.celedon@postgrado.uv.cl

Received: 14 April 2025
Accepted: 12 June 2025

Abstract

Context. Double periodic variables (DPVs) are a group of semi-detached interacting binaries that exhibit a long photometric cycle with an average length of approximately 33 times the orbital period of the system. It has been proposed that this long photometric cycle originates from a modulated mass transfer rate from the donor star, which itself is driven by an internal magnetic dynamo within the donor. One of the most well-studied DPVs in the Milky Way is AU Monocerotis (AU Mon).

Aims. We aim to enhance our understanding of the origin of the long photometric cycle in AU Mon by characterising its behaviour through the analysis of available photometric data from several databases and surveys.

Methods. We summarise previous findings on the system and analyse its published multi-wavelength photometry from different sources, covering 46.3 years, to study the variability of its light curve.

Results. We find that the orbital period has remained constant over recent decades, but the long cycle of approximately 417 days vanished around 2010. From an O-C analysis, we conclude that the system is experiencing a change in its orbital period of no greater than 0.038 ± 0.040 s yr⁻¹, and thus, imposing a value of 2 × 10⁻⁸ M_⊙ yr⁻¹ for Ṁ in a fully conservative mass transfer regime. The disappearance of the long cycle is more evident in the V filter than in the Ic filter. In the latter, a small amplitude variation related to the long cycle is still detected. A time-series analysis of the disentangling light curve in the Ic filter shows a transient periodicity of approximately 1910 days lasting at least 2000 days before it also disappears around the year 2020. An analysis of the available AAVSO Photometric All-Sky Survey light curves around the year 2013 shows a strong periodicity at approximately 280 days, which appears to be stronger in the Z filter.

Conclusions. We report what is the second observation of the sudden disappearance of the long cycle in a DPV, after the Galactic DPV TYC 5353-1137-1. The disappearance of the long cycle in AU Mon is a strong constraint for current models that aim to explain the long cycle in DPVs.