A new study of the spectroscopic binary 7 Vul with a Be star primary^⋆

¹ Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
e-mail: hec@sirrah.troja.mff.cuni.cz
² Astronomical Institute, Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic
³ Hvar Observatory, Faculty of Geodesy, Zagreb University, Kačićeva 26, 10000 Zagreb, Croatia
⁴ Instituto de Astronomia, Geofísica e Ciencias Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP, Brazil
⁵ Physics & Astronomy Department, University of Victoria, PO Box 3055 STN CSC, Victoria, BC V8W 3P6, Canada
⁶ Undergraduate student, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 180 00 Praha 8, Czech Republic

Received: 15 March 2020
Accepted: 11 May 2020

Abstract

We confirmed the binary nature of the Be star 7 Vul, derived a more accurate spectroscopic orbit with an orbital period of 69_.^d4212±0_.^d0034, and improved the knowledge of the basic physical elements of the system. Analyzing available photometry and the strength of the Hα emission, we also document the long-term spectral variations of the Be primary. In addition, we confirmed rapid light changes with a period of 0_.^d5592, which is comparable to the expected rotational period of the Be primary, but note that its amplitude and possibly its period vary with time. We were able to disentangle only the He I 6678 Å line of the secondary, which could support our tentative conclusion that the secondary appears to be a hot subdwarf. A search for this object in high-dispersion far-UV spectra could provide confirmation. Probable masses of the binary components are (6 ± 1) 𝓜_⊙^N and (0.6 ± 0.1) 𝓜_⊙^N. If the presence of a hot subdwarf is firmly confirmed, 7 Vul might be identified as a rare object with a B4-B5 primary; all Be + hot subdwarf systems found so far contain B0-B3 primaries.