Mirror eclipses in the cataclysmic variable IP Peg

We present time resolved K-band infrared spectra of the dwarf nova (DN) IP Peg in early quiescence. The Brackett gamma and He I (lambda2.058 1) 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 acc retion disc. Mirror eclipses are a direct probe of the structure and physic al conditions of accretion discs. For example, on assuming the relevant lev el populations to be in local thermal equilibrium, we constrain the tempera ture and density of the optically thin material, causing the mirror eclipse in IP Peg to be 10 000 less than or similar to T less than or similar to 2 0 000 K and rho similar to 10(-11) g cm(-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 di sc models are examined.