Photometric study of hot Algol-type binaries with long cycles

¹ Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
² Astronomical Observatory, Volgina 7, 11060 Belgrade 38, Serbia
³ Issac Newton Institute of Chile, Yugoslavia Branch, 11060 Belgrade, Serbia
⁴ Centro de Investigación en Ciencias del Espacio y Física Teórica, Universidad Central de Chile, Av. Francisco de Aguirre 0405, La Serena, Chile
⁵ Instituto de Física y Astronomía, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
⁶ Departamento de Tecnologías Industriales, Faculty of Engineering, Universidad de Talca, Merced 437, Curicó, Chile
⁷ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Rome, Italy
⁸ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland

^⋆ Corresponding author: juangarces@udec.cl

Received: 14 November 2024
Accepted: 9 July 2025

Abstract

Context. Double periodic variables (DPVs) are hot Algol-type interacting binary systems with an orbital and a long photometric cycle. The origin of the latter may be related to cyclic structural changes in the accretion disc that surrounds the gainer star that are driven by a variable mass-transfer rate. If this is the case, changes in the orbital light curve would be expected throughout the long cycle.

Aims. We conducted a detailed photometric analysis of the light curves of 134 Large Magellanic Cloud (LMC) DPVs to investigate variations in the morphology of the orbital light curves as a function of the long-cycle phase.

Methods. We separated the two photometric cycles from the Optical Gravitational Lensing Experiment (OGLE) I band light curves for the systems. We thus compared the orbital light curves at opposite long-cycle phases, investigated the stability of the long period, and analysed the residuals of the separation process to search for significant frequencies above a 1% false-alarm probability threshold.

Results. We confirm that the DPVs OGLE-LMC-DPV-097 and OGLE-BLG-ECL-157529 change most strongly in their orbital light curves throughout the long cycle. By comparison, about 50% of the sample exhibits moderate morphological variations, in particular, around orbital phase 0.5. This is likely associated with structural changes in the accretion discs. In addition, we identified 18 DPVs with variable long periods, including 10 new cases. In some of them, the long period either increases or decreases continuously over time. For the first time, we found DPV systems that alternate between the two behaviours at different epochs. Moreover, we detected frequencies in the residuals that might be directly related to changes in the morphology of the orbital curves. Finally, some previously reported frequencies disappear when a variable long period is taken into account.