The IACOB project

XVI. Surface helium abundances in Galactic O-type stars: Indications for identifying binary interaction products

¹ Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
³ Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), c. Martí i Franquès, 1, 08028 Barcelona, Spain
⁴ European Southern Observatory, Alonso de Córdova 3107 Vitacura, Santiago, Chile
⁵ Argelander Institut für Astronomie, Auf dem Hügel 71, DE-53121 Bonn, Germany
⁶ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, A-6020 Innsbruck, Austria
⁷ Instituto de Astrofísica de La Plata, Facultad de Ciencias Astronómicas y Geofísicas, CONICET–UNLP, Paseo del Bosque s/n, La Plata, Argentina
⁸ LMU Munich, Universitätssternwarte, Scheinerstrasse 1, 81679 München, Germany
⁹ Centro de Astrobiología, CSIC-INTA, Crtra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz (Madrid), Spain
¹⁰ School of Mathematics, Statistics and Physics, Newcastle University, UK
¹¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, DE-53121 Bonn, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 January 2026
Accepted: 29 April 2026

Abstract

The presence of massive O-type stars with surfaces enriched by CNO-cycle products has been known since the early 1980s. For many years, internal rotational mixing was assumed to be the dominant mechanism responsible for this chemical contamination. However, accumulating evidence suggests that binary interaction may play an equally important, if not dominant, role.