[vsnet-newvar 2576] SDSS J121209.31+013627.7

[Taichi Kato]

   EF Eri-like object in "permanent" low state?
   magnetic CV with a brown-dwarf binary?
   probably worth monitoring for long-term activity

http://xxx.lanl.gov/abs/astro-ph/0607389

The nature of the close magnetic white dwarf + probable brown dwarf 
binary SDSS J121209.31+013627.7 

Authors: M. R. Burleigh (1), T. R. Marsh (2), B. T. Gansicke (2), M. 
R. Goad (1), V. Dhillon (3), S. P. Littlefair (3), M. Wells (4), N. 
P. Bannister (1), C. P. Hurkett (1), A. Martindale (1), P. D. Dobbie 
(1), S. L. Casewell (1), D. E. A. Baker (1), J. Duke (1), J. Farihi 
(5), M. J. Irwin (6), P. C. Hewett (6), P. Roche (7), F. Lewis (7) 
((1) Department of Physics and Astronomy, University of Leicester (2) 
Department of Physics, University of Warwick (3) Department of 
Physics and Astronomy, University of Sheffield, Sheffield, UK, (4) 
Oundle School, Northamptonshire, UK (5) Gemini Observatory, USA, (6) 
Institute of Astronomy, University of Cambridge, UK, (7) Department 
of Physics and Astronomy, University of Wales, UK) Comments: 
Submitted to MNRAS 

Optical time series photometry of the short period magnetic white 
dwarf + probable brown dwarf binary SDSS 121209.31+013627.7 reveals 
pulse-like variability in all bands from i' to u', peaking at u'. 
These modulations are most likely due to a self-eclipsing accretion 
hot spot on the white dwarf, rotating into view every 88.43 minutes. 
This period is commensurate with the radial velocity period 
determined by Schmidt et al. 2005 of ~90 minutes, and consistent with 
the rotation period of the accretor being equal to the binary orbital 
period. We combine our observations with those recently published by 
Koen and Maxted 2006 to provide an accurate ephemeris. We also detect 
the system in X-rays with Swift, and estimate the accretion rate at 
~1x10^-13Msun per year. We suggest that SDSS1212 is most likely a 
magnetic cataclysmic variable in an extended state of very low 
accretion, similar to the well-studied Polar EF Eri. Alternatively, 
the putative brown dwarf is not filling its Roche Lobe and the system 
is a detached binary in which the white dwarf is efficiently 
accreting from the wind of the secondary. Six such post-common 
envelope, "pre-Polar" systems - termed "low accretion rate Polars 
(LARPs)" by Schwope et al. 2002 - have previously been identified 
through optical cyclotron emission lines. Cyclotron emission from 
SDSS1212 has recently been detected in the near-IR Debes et al. 2006 
but, if detached, it would be the first "LARP" with a probably 
sub-stellar secondary. It is unclear whether an L-dwarf wind is 
strong enough to provide the measured accretion rate. We suggest 
further observations to distinguish between the Roche Lobe over-flow 
and wind accretion scenarios.