Photometric identification of the periods of the first candidate extragalactic magnetic massive stars^⋆

¹ Groupe d’Astrophysique des Hautes Énergies, Institut d’Astrophysique et de GéophysiqueDepartment AGO, Université de Liège, 19c Allée du 6 Août, B5c, 4000 Sart Tilman, Belgium
e-mail: naze@astro.ulg.ac.be
² Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA ^⋆⋆⋆
4 Department of Physics, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, ON K7K 4B4, Canada
⁵ Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland

Received: 12 February 2015
Accepted: 19 March 2015

Abstract

Galactic stars belonging to the Of?p category are all strongly magnetic objects exhibiting rotationally modulated spectral and photometric changes on timescales of weeks to years. Five candidate Of?p stars in the Magellanic Clouds have been discovered, notably in the context of ongoing surveys of their massive star populations. Here we describe an investigation of their photometric behaviour, revealing significant variability in all studied objects on timescales of one week to more than four years, including clearly periodic variations for three of them. Their spectral characteristics along with these photometric changes provide further support for the hypothesis that these are strongly magnetized O stars, analogous to the Of?p stars in the Galaxy.