Superhumps in systems with intermediate mass ratios

Superhumps have been observed in systems with orbital periods up to 3.5 hours and with mass ratios significantly larger than 1/4. Previous theoretical arguments had led us to believe that an eccentric disc could only be excited if q≲0.25. We demonstrate that it is possible for an accretion disc in a system with intermediate mass ratio to expand to the 3:1 eccentric inner Lindblad resonance during periods of reduced mass transfer from the secondary star. Such episodic reductions in Ṁ_sec are characteristic of the VY Sculptoris stars, which all have periods in the 3-4 hour range. Disc simulations show that eccentricity can be thus excited in systems with q<1/3. We review the theoretical estimates for the precession rate of an eccentric gaseous disc. We show that whilst a simple dynamical estimate is inadequate to describe the observations, an estimate that accounts for pressure forces is consistent with the observations and the requirement that the mass ratio be less than 1/3 for the resonance to be excited.