The field high-amplitude SX Phe variable BL Cam: results from a multisite photometric campaign^*

II. Evidence of a binary – possibly triple – system

¹ Observatoire du Bois de Bardon, 16110 Taponnat, France e-mail: stephane.fauvaud@wanadoo.fr
² Groupe Européen d'Observation Stellaire (GEOS), 23 Parc de Levesville, 28300 Bailleau l'Evêque, France
³ Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France, and Observatoire de Paris-Meudon, LESIA, 92190 Meudon, France e-mail: jean-pierre.sareyan@obspm.fr
⁴ Institut de Ciéncies de l'Espai (CSIC-IEEC), Campus UAB, Facultat de Ciéncies, Torre C5, parell, 2a pl., 08193 Bellaterra, Spain
⁵ Instituto de Astrofísica de Andalucía, CSIC, PO Box 3004, 18080 Granada, Spain
⁶ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium
⁷ Variable Star Section, Norwegian Astronomical Society, PO Box 1029 Blindern, 0315 Oslo, Norway
⁸ Sulphur Flats Observatory, 449 Sulphur Creek Road, Jemez Springs, NM 87025, USA
⁹ Martin Observatory, Physics Dep., Virginia Polytechnic Institute & State Univ., Blacksburg, VA 24061, USA
¹⁰ Astronomical Institute, Charles University Prague, V Holesovickach 2, 180 00 Praha 8, Czech Republic
¹¹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
¹² Observatorio Astronómico Nacional, Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 877, Ensenada, BC 22860, Mexico
¹³ Facultad de Ciencias Químicas e Ingeniería, Universidad Autónoma de Baja California, Calzada Tecnologíco No. 14418, Mesa de Otay, C.P. 22390, Tijuana, B.C. Mexico
¹⁴ Societá Astronomica Le pleiadi, Italy
¹⁵ Observatoire des trois korrigans, 85600 Treize-Septiers, France
¹⁶ Department of Physics and Astronomy, Brigham Young University, N283 ESC, Provo, UT 84602, USA
¹⁷ Beersel Hills Observatory, B-Beersel, Belgium
¹⁸ Akdeniz University, Department of Physics, 07058 Antalya, Turkey
¹⁹ Ataturk University, Department of Physics, 25340 Erzurum, Turkey
²⁰ American Association of Variable Star Observers, 25 Birch St., Cambridge, MA 02138, USA
²¹ Project Eridanus, Observatory and Planetarium of Johann Palisa, Tr. 17. listopadu 15, 708 33 Ostrava-Poruba, Czech Republic
²² Club d'Astronomie Lyon Ampère, 37 rue Paul Cazeneuve, 69008 Lyon, France
²³ Club Eclipse, 22 rue du Borrégo, Appt 5A4, 75020 Paris, France
²⁴ Astronomical Institute, Academy of Sciences, 251 65 Ondrejov, Czech Republic
²⁵ Ege University Observatory, 35100 Bornova, Izmir, Turkey
²⁶ Vesqueville Observatory, rue de Fayet 8, 6870 Vesqueville, Belgium
²⁷ Geneva Observatory, 1290 Sauverny, Switzerland

Received: 11 February 2010
Accepted: 21 February 2010

Abstract

Context. Short-period high-amplitude pulsating stars of Population I (δ Sct stars) and II (SX Phe variables) exist in the lower part of the classical (Cepheid) instability strip. Most of them have very simple pulsational behaviours, only one or two radial modes being excited. Nevertheless, BL Cam is a unique object among them, being an extreme metal-deficient field high-amplitude SX Phe variable with a large number of frequencies. Based on a frequency analysis, a pulsational interpretation was previously given.

Aims. We attempt to interpret the long-term behaviour of the residuals that were not taken into account in the previous Observed-Calculated (O–C) short-term analyses.

Methods. An investigation of the O–C times has been carried out, using a data set based on the previous published times of light maxima, largely enriched by those obtained during an intensive multisite photometric campaign of BL Cam lasting several months.

Results. In addition to a positive (161 ± 3) × 10^-9 yr^-1 secular relative increase in the main pulsation period of BL Cam, we detected in the O–C data short- (144.2 d) and long-term (~3400 d) variations, both incompatible with a scenario of stellar evolution.

Conclusions. Interpreted as a light travel-time effect, the short-term O–C variation is indicative of a massive stellar component (0.46 to 1 ) with a short period orbit (144.2 d), within a distance of 0.7 AU from the primary. More observations are needed to confirm the long-term O–C variations: if they were also to be caused by a light travel-time effect, they could be interpreted in terms of a third component, in this case probably a brown dwarf star (≥0.03 ), orbiting in ~3400 d at a distance of 4.5 AU from the primary.