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Astron. Astrophys. 348, 364-370 (1999)

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1. Introduction

The secular evolution of cataclysmic variable stars (CVs) is driven by mass transfer from the secondary star onto the primary star due to angular momentum loss from the binary system. The period gap is a region of the orbital period distribution, roughly between 2-3 hr, practically void of CVs. The main cause of the angular momentum loss above the orbital gap seems to be magnetic breaking of a stellar wind escaping from the secondary star. Below the period gap, gravitational radiation seems to be the dominant cause (e.g. Howell et al. 1997, Kolb 1993). Accordingly, the mass ratio changes when the binary evolves from long periods to ultra-short periods. For example, only low mass ratios ([FORMULA]) are found below the period gap and the ultra-short period systems apparently host two different populations of CVs (Howell et al. 1997). One of these populations consists of systems approaching the minimum period from the longer period side and the other consists of systems which have "bounced-off" from the period minimum and now are evolving to longer periods but with degenerate (brown dwarf-like) secondaries having masses between 0.02 an 0.06 [FORMULA] and radii near 0.1 [FORMULA] (Howell et al. 1997). These low-luminosity systems are interesting because they can be used to set a lower limit on the age of the Galaxy (Howell et al. 1997). In addition, they provide a unique laboratory to test models of late evolution stages of red dwarfs in close binary systems. Four stars were identified as candidate "period bouncers" by Patterson (1998), whereas Howell et al. (1997) speculate that at least some of the large amplitude dwarf novae are post-period-minimum cataclysmic variables. In general, these "period bouncers" should be hard to detect due to the faintness of their secondaries and the low brightness and long-recurrence time of their accretion disks associated with extreme low mass accretion rates ([FORMULA]). In this paper we deal with an empirical relation potentially useful to separate the aforementioned populations of ultra-short period CVs, identifying the "period-bouncers" as the lowest q systems. Surprisingly, we found indirect evidence for distinct disk properties in dwarf novae above and below the period gap. A review of CVs including dwarf novae and SU UMa stars is given by Warner (1995).

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© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999