HD 173977: An ellipsoidal δ Scuti star variable ^*,**

¹ Observatoire de la Côte d'Azur, Département GEMINI – UMR 6203, BP 4229, 06304 Nice Cedex 4, France e-mail: [eric;mathias]@obs-nice.fr
² Instituto de Astrofisica de Andalucia, Apt. 3004, 18080 Granada, Spain
³ Departamento de Astronomia y Meteorologia,   Universitat de Barcelona, 08028 Barcelona, Spain
⁴ Instituto de Astronomia de la UNAM, Ap. P. 877, Ensenada, BC, Mexico
⁵ Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, Apdo. Postal 70-264, Mexico DF 04510, Mexico

Received: 10 July 2003
Accepted: 23 June 2004

Abstract

We present spectroscopic and photometric observations of the star HD 173977. It appears that the star is part of a double line binary system, with a period of 1.801 d, corresponding to twice the period of the photometric variations. Hence the star is an ellipsoidal variable. The system is probably synchronized. The physical parameters of both components were derived through two independant methods, one based on evolutionary tracks, the other being the result of the behaviour of light curves in a close binary system. After removing the ellipsoidal variations, 3 frequencies are detected in the photometric data: 8.56, 14.51 and 16.42 d^-1, while 2 additional frequencies are also possible: 10.96 and 12.11 d^-1. In accordance with its position in the HR diagram, the primary component of HD 173977 should be considered as a δ Scuti star and no longer as a γ Doradus star. 
In addition, HD 173844, used as a check star, is discovered variable with a 15.79 d^-1 frequency and is classified as a δ Scuti star.