2001MNRAS.327..475L

We present time resolved K-band infrared spectra of the dwarf nova (DN) IP Peg in early quiescence. The Brackett γ and Hei (λ2.0581) lines in our data show hitherto unseen behaviour, which we term a mirror eclipse, and interpret as an eclipse of the secondary star by an optically thin accretion disc. Mirror eclipses are a direct probe of the structure and physical conditions of accretion discs. For example, on assuming the relevant level populations to be in local thermal equilibrium, we constrain the temperature and density of the optically thin material, causing the mirror eclipse in IP Peg to be 10000≲T≲20000K and rho∼10^–11gcm^–3 respectively. In order to match our data, we find that at least the outermost 20 per cent of the disc (in radius) must be entirely optically thin. Implications for time-dependent disc models are examined.