Spectroscopic orbit and variability of the Be star V1624 Cyg = 28 Cyg^⋆

¹ Astronomical Institute of Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, CZ-180 00 Praha 8 – Troja, Czech Republic
² Physics & Astronomy Department, University of Victoria, PO Box 3055 STN CSC Victoria, BC V8W 3P6, Canada
³ Astronomical Institute, Czech Academy of Sciences, CZ-251 65 Ondřejov, Czech Republic
⁴ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, 5 place Jules Janssen, 92195 Meudon, France
⁵ Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kačićeva 26, 10000 Zagreb, Croatia
⁶ Astronomy and Space Sci. Dept., Science Faculty, Erciyes University, 38039 Kayseri, Türkiye

^⋆⋆ Corresponding author: Petr.Harmanec@matfyz.cuni.cz

Received: 19 February 2025
Accepted: 19 May 2025

Abstract

In recent years the idea, first formulated many decades ago, that the Be phenomenon could be causally related to the duplicity of Be stars, has been repeatedly reconsidered from various perspectives. It is important, therefore, to have reliable information on Be stars, which are confirmed members of binary systems. This study is devoted to V1624 Cyg = 28 Cygni, which was recently identified as a binary with a compact secondary. By measuring the radial velocities (RVs) of the wings of the H alpha emission line and using archival data and published RVs from the International Ultraviolet Explorer, we demonstrate that the Be primary moves in the 359.​​^d26 orbit found recently from interferometry. Our preliminary radial-velocity solution leads to binary masses of 5.6 M_⊙, and 0.66 M_⊙. Moreover, we documented large and irregular spectral, brightness, and colour changes over a time interval of several decades to show that the object never completely lost its circumstellar matter.