tokens: an order of magnitude lower just by increasing the disc viscosity by a factor of 3–6 (Stehle 1999 ). Therefore, while increasing the disc temperatures increases the opening angle of the resulting spiral arms (e.g. Makita, Miyawaki Matsuda 2000 , and references therein), increasing the disc viscosity seems to have a similar (and seemingly stronger) effect. The point we make here is that wide open spiral arms may be more a signature of a high viscosity disc than of a hot disc, and that a large disc viscosity might lead to detectable, wide open tidally induced spiral density waves even when the disc is relatively cool and small. Thus, the detection of spiral arms in V2051 Oph during quiescence suggests that its quiescent accretion disc has high viscosity. This is in line with the results of Baptista Bortoletto ( 2004 ) – which inferred a high viscosity α ≃ 0.1−0.2 for the quiescent accretion disc of V2051 Oph – and with those of Baptista et al. ( 2007 ) – which suggest that the outbursts of V2051 Oph are the result of bursts of enhanced mass transfer on to a high-viscosity accretion disc. 5 SUMMARY Infrared JHK s quiescent light curves of V2051 Oph after removal of the ellipsoidal contribution from its mass donor star show an out-of-eclipse double-wave modulation the amplitude of which increases with wavelength. Eclipse maps derived from these light curves reveal two asymmetric arcs extended in azimuth at the intermediate/outer disc regions, which are reminiscent of those seen in eclipse maps of the dwarf nova IP Peg in outburst and are similarly interpreted as tidally induced spiral density waves in the V2051 Oph accretion disc. In agreement with this interpretation, the maxima of the double-wave modulation coincide with the binary phases where the spiral arms are seen face-on. The spiral arms are located at different distances from disc centre; the inner arm is at an average radius of (at a corresponding Keplerian velocity of ), while the outer arms is at . We estimate the opening angle of these spirals to be θ s = 21° ± 4°. The detection of wide open spiral arms in V2051 Oph during quiescence suggests that its quiescent accretion disc has high viscosity. Broad-band JHK s disc spectra were extracted for concentric annular disc regions. The inner disc atmosphere () is made of hot and tenuous gas ( and surface densities ), suggesting that either an extended hot and tenuous chromosphere veils the emission from an underlying, colder accretion disc or that the inner disc itself has evaporated into a hot and tenuous corona. The disc gas becomes cold and opaque in its outer regions (). The spiral arms are hotter and have lower column densities than the disc at same radial range, with differences within the 2σ confidence level. The JHK s best-fitting hydrogen continuum emission models are in very good agreement with the B -band observations of Baptista Bortoletto ( 2004 ), suggesting that the inner disc regions were also optically thin and with
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