The AH Pictoris syndrome: Continuous trains of stunted outbursts in novalike variables

Abstract

Novalike variables are a subgroup of cataclysmic variables (CVs) that -- unlike dwarf novae -- do not exhibit strong brightenings in their long-term light curves. Variations over time scales of weeks, months or years are mostly restricted to irregular low-amplitude modulations. However, some of them occasionally suffer from so-called stunted outbursts, that is, small-scale brightenings of less than a magnitude lasting for a couple of days to weeks. There is no consensus about the physical mechanisms behind these outbursts. Here I discuss the common properties of a group of novalike variables (which I call AH~Pictoris stars after its most prominent member) that exhibit a continuous train of successive stunted outbursts over their entire observational history, or at least for several years. The outburst amplitudes are stable in a given system, always ranging between 0.5 and 1~mag in the visual band. The outburst intervals, at an overall range between 12 and 30 days, and the outburst profiles can gradually evolve, but no sudden changes are observed. On shorter time scales the orbital waveforms are not only surprisingly similar, but also evolve in the same way over the outburst cycle. All AH~Pic stars have absolute visual magnitudes in the overlap region between absolute magnitudes of all novalike variables and of quiescent dwarf novae above the CV period gap. So far, I identified seven novalike variables with the consistent photometric behavior which may be termed the AH~Pic syndrome. Several more systems may be related objects. The relationship to the anomalous Z~Cam stars is discussed.