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Astron. Astrophys. 352, 239-247 (1999)

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1. Introduction: about RZ Leonis

Dwarf novae are interacting binary stars in which a Roche-lobe filling main-sequence secondary looses mass through the [FORMULA] point. The transferred mass falls along a ballistic trajectory towards the heavier white dwarf primary, forming an accretion disk. The disk undergoes semi-periodic collapse during which matter is accreted by the compact primary. The result is a release of gravitational energy which is observed as a system brightening. This is called a dwarf nova outburst. Dwarf novae, a subclass of cataclysmic variable stars (CVs), have been reviewed by Warner (1995a).

RZ Leonis is a long cycle-length large-amplitude dwarf nova with only 7 outbursts recorded since 1918 (e.g Vanmuster & Howell 1996) and with an estimated distance from earth between 174 and 246 pc (Sproats et al. 1996).

Humps in the light curve of RZ Leo repeating with a [FORMULA] period were observed by Howell & Szkody (1988). They concluded that this dwarf nova is a candidate for SU UMa star probably seen under a large inclination. This assumption is supported by the finding of broad double emission-lines in the optical spectrum (Cristiani et al. 1985, Szkody & Howell 1991). Orbital humps are observed in some high-inclination dwarf novae (e.g. Szkody 1992), they probably reflect the pass of the disk-stream interacting region (often named hot spot or bright spot) along the observer's line of sight.

The study of the hot spot variability of RZ Leo is potentially useful to constrain models of gas dynamics in close binary systems. In the classical view, the hot spot is formed during the shock interaction of matter in the gaseous stream flowing from [FORMULA] (the inner Lagrangian point) with the outer boundary of the accretion disk. This picture was consistent with photometric observations of dwarf novae during many years. However, this view conflicts with recents observations indicating anomalous hot spots in many systems. For example, in many cases, Doppler tomography does not show the effect of a hot spot at all, or indicates that the hot spot is not in the place where we would expect a collision between the gaseous stream and the outer boundary of the disk (e.g. Wolf et al. 1998). To explain these findings, the possibility of gas stream overflow has been worked out. In this view the hot spot is formed behind the white dwarf by the ballistic impact of a deflected stream passing over the white dwarf (e.g. Armitage & Livio 1998, Hessmann 1999). However, this scenario has not yet been confirmed by observations. Furthermore, recent three-dimensional numerical simulations indicate the absence of a shock between the stream and the disk. The interaction between the stream and the common envelope of the system forms and extended shock wave along the edge of the stream, whose observational properties are roughly equivalent to those of a hot spot in the disk (Bisikalo et al. 1998).

This paper is aimed to confirm the reported photometric period and to establish a long-term hump ephemeris. We also expect to detect systematic luminosity trends and get insights about the hump variability and hot spot nature. Interestingly, we find some phenomena conflicting, in many ways, with the classical scenario of the hot spot forming region.

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© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999