SDSS J162520.29+120308.7 – a new SU Ursae Majoris star in the period gap^⋆

¹ Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-176 Warszawa, Poland
e-mail: olech@camk.edu.pl
² Departamento de Fisica Aplicada, Facultad de Ciencias Experimentales, Universidad de Huelva, 21071 Huelva, Spain
³ Center for Backyard Astrophysics, Observatorio del CIECEM, Parque Dunar, Matalascanas, 21760 Almonte, Huelva, Spain
⁴ Dept. of Physics and Astronomy, 6127 Wilder Laboratory, Dartmouth College, Hanover, NH 03755-3528, USA
⁵ TUBITAK National Observatory, Akdeniz University Campus, 07058 Antalya, Turkey
⁶ Institute of Computer Science, Faculty of Civil Engineering, Brno University of Technology, 602 00 Brno, Czech Republic
⁷ The Virtual Telescope Project, via Madonna del Loco 47, 03023 Ceccano (FR), Italy
⁸ Physics Department, Rochester Institute of Technology, Rochester, New York 14623, USA
⁹ American Association of Variable Star Observers, 49 Bay State Rd., Cambridge, MA 02138, USA
¹⁰ Center for Backyard Astrophysics (Flanders), American Association of Variable Star Observers (AAVSO), Alan Guth Observatory, Koningshofbaan 51, Hofstade, Aalst, Belgium
¹¹ Center for Backyard Astrophysics (Pukekohe), New Zealand
¹² 6025 Calle Paraiso, Las Cruces, NM 88012, USA
¹³ Istanbul University, Faculty of Sciences, Department of Astronomy and Space Sciences, 34119 University, Istanbul, Turkey
¹⁴ Silver Lane, West Challow, Wantage, OX12 9TX, UK
¹⁵ The British Astronomical Association, Variable Star Section (BAA VSS), Burlington House, Piccadilly, London, W1J 0DU, UK
¹⁶ Center for Backyard Astrophysics (Colorado), Antelope Hills Observatory, 980 Antelope Drive West, Bennett, CO 80102, USA
¹⁷ Department of Astronomy, Columbia University, New York, NY 10027, USA

Received: 11 January 2011
Accepted: 10 May 2011

Abstract

We report results of an extensive world-wide observing campaign devoted to the recently discovered dwarf nova SDSS J162520.29+120308.7 (SDSS J1625). The data were obtained during the July 2010 eruption of the star and in August and September 2010 when the object was in quiescence. During the July 2010 superoutburst, SDSS J1625 clearly displayed superhumps with a mean period of P_sh = 0.095942(17) days (138.16 ± 0.02 min) and a maximum amplitude reaching almost 0.4 mag. The superhump period was not stable, decreasing very rapidly at a rate of Ṗ =  −1.63(14) × 10^-3 at the beginning of the superoutburst and increasing at a rate of Ṗ = 2.81(20) × 10^-4 in the middle phase. At the end of the superoutburst, it stabilized around the value of P_sh = 0.09531(5) day.

During the first twelve hours of the superoutburst, a low-amplitude double wave modulation was observed whose properties are almost identical to early superhumps observed in WZ Sge stars. The period of early superhumps, the period of modulations observed temporarily in quiescence, and the period derived from radial velocity variations are the same within measurement errors, allowing us to estimate the most probable orbital period of the binary to be P_orb = 0.09111(15) days (131.20 ± 0.22 min). This value clearly indicates that SDSS J1625 is another dwarf nova in the period gap. Knowledge of the orbital and superhump periods allows us to estimate the mass ratio of the system to be q ≈ 0.25. This high value poses serious problems for both the thermal and tidal instability (TTI) model describing the behaviour of dwarf novae and for some models explaining the origin of early superhumps.