Superhumps in systems with intermediate mass ratios

Superhumps have been observed in systems with orbital periods up to 3.5 hou rs and with mass ratios significantly larger than 1/4. Previous theoretical arguments had led us to believe that an eccentric disc could only be excit ed if q less than or similar to 0.25. We demonstrate that it is possible fo r an accretion disc in a system with intermediate mass ratio to expand to t he 3:1 eccentric inner Lindblad resonance during periods of reduced mass tr ansfer from the secondary star. Such episodic reductions in (M)over dot(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 exc ited in systems with q < 1/3. We review the theoretical estimates for the p recession rate of an eccentric gaseous disc. We show that whilst a simple d ynamical estimate is inadequate to describe the observations, an estimate t hat accounts for pressure forces is consistent with the observations and th e requirement that the mass ratio be less than 1/3 for the resonance to be excited. (C) 2000 Elsevier Science B.V. All rights reserved.