Excess mid-infrared emission in cataclysmic variables

We present a search for excess mid-infrared emission due to circumbinary (CB) material in the orbital plane of cataclysmic variables (CVs). Our motivation stems from the fact that the strong braking exerted by a CB disc on the binary system could explain several puzzles in our current understanding of CV evolution. Since theoretical estimates predict that the emission from a CB disc can dominate the spectral energy distribution (SED) of the system at λ 5 μm, we obtained simultaneous visible to mid-infrared (mid-IR) SEDs for eight systems. We report detections of SS Cyg at 11.7 μm and AE Aqr at 17.6 μm, both in excess of the contribution from the secondary star. In AE Aqr, the IR likely originates from synchrotron-emitting clouds propelled by the white dwarf. In SS Cyg, we argue that the observed mid-IR variability is difficult to reconcile with simple models of CB discs and we consider free-free emission from a wind. In the other systems, our mid-IR upper limits place strong constraints on the maximum temperature of a putative CB disc. The results show that if any sizeable CB discs are present in these systems, they must be self-shadowed or perhaps dust-free, with the peak thermal emission shifted to far-IR wavelengths.