Spectroscopic Studies of 30 Short-period Cataclysmic Variable Stars, and Remarks on the Evolution and Population of Similar Objects
We present spectroscopy and orbital periods Porb for 30 apparently non-magnetic cataclysmic binaries with periods below about 3 hours, nearly all of which are dwarf novae, mostly of the SU Ursae Majoris subclass. We then turn to the evidence supporting the prediction that short-period dwarf novae evolve toward longer periods after passing through a minimum period -- the "period bounce" phenomenon. Plotting data from the literature reveals that for superhump period excess $\epsilon = (P_{\rm sh} - P_{\rm orb} )/P_{\rm orb}$ below about 0.015, the period appears to increase with decreasing epsilon, agreeing at least qualitatively with the predicted behavior. Next, motivated by the long (decadal) outburst intervals of the WZ Sagittae subclass of short-period dwarf novae, we ask whether there could be a sizable population of "lurkers" -- systems that resemble dwarf novae at minimum light, but do not outburst over accessible timescales (or at all), and therefore do not draw attention to themselves. By examining the outburst history of the Sloan Digital Sky Survey sample of CVs, which were selected by color and not by outburst, we find that a large majority of the color-selected dwarf-nova-like objects have been observed to outburst, and conclude that "lurkers", if they exist, are a relatively minor part of the CV population.