The dwarf nova RZ Leonis : photometric period, "anti-humps" and normal alpha disk

Abstract

We present results of differential photometry of the dwarf nova RZ Leonis spanning a 11-year baseline. The most striking feature of the light curve is a non-coherent periodic hump of variable amplitude. A seasonal time series analysis yields a photometric period of 0.0756(12) d. In addition, low amplitude fluctuations of the mean magnitude in time scale of months are observed. We find that the hump's amplitude is anti-correlated with the star's mean magnitude and becomes "negative" (i.e. an absorption feature or "anti-hump") when the system is very faint. Secondary humps and "anti-humps" are also observed. The transition from "anti-humps" to fully developed humps occurs on a time scale of 70 days. We interprete the observations as a rapid response of the accretion disk to the increase of mass transfer rate. In this case we deduce a viscosity parameter alpha ~ 0.08, i.e. much larger than often claimed for WZ Sge-like stars. We note that the secondary star in RZ Leo is close to a main-sequence red dwarf and not a brown-dwarf like star as suggested for other long cycle-length SU UMa stars like WZ Sge and V592 Her. Our results indicate that large amplitude and long cycle length dwarf novae might not necessarily correspond to objects in the same evolutive stage.