The Missing Cv Population: Results from an Objective Prism Survey

We present the results of a search for cataclysmic variables (CVs) in the Calán-Tololo survey. We detected a total number of 21 CVs, 12 of them are previously unknown objects. Our results suggest that the mismatch between the theoretically predicted sample and the observed one is not due to observational bias but has to be resolved by a revision of the theoretical models. 1. The survey Theoretical models of the CV population predict that the vast majority consists of low-mass-transfer systems, which have passed a theoretical minimum period P min,the ∼ 65 min and are evolving back to longer periods. Observationally, this should cause a pileup of systems at P min,the (Stehle, Kolb, & Ritter 1997). However, the observed minimum period is P min,obs ∼ 78 min, and neither the large number of evolved CVs nor the pileup are seen (Patterson 1998). CVs of this 'missing population' are expected to be intrinsically very faint and to have long outburst recurrence times, making them hard to discover. The presently observed sample is likely to be biased towards bright, young CVs, and does not serve as an unambiguous testbed for theoretical models. Spectrocopic surveys should be ideally suited to discover evolved CVs, as low mass-transfer-rate systems generally show strong emission lines. The Calán-Tololo objective prism survey (hereafter CTS; Maza et al. 1989) covers 5150 deg 2 of the southern sky with |b| ≥ 20 • down to ∼18.5 mag. About half of the plates have been examined, and 59 candidate CVs have been selected visually on the basis of their spectral appearance on the plates. Follow-up observations included medium-resolution spectroscopy, calibrated and time-series photometry for all previously unknown objects, and time-resolved spectroscopy for selected identified CVs.