Short-per cataclysmic var. Balmer dec.

We investigate whether period-bounce CVs can be distinguished from short-period pre-bounce CVs through their Balmer decrements using optical spectra from the Sloan Digital Sky Survey (SDSS). Differences in their Balmer decrements are expected to trace the distinct physical conditions in the accretion discs of period-bouncers resulting from their lower mass-transfer rates. Two samples of non-magnetic CVs with public SDSS optical spectra were constructed: one of short-period pre-bounce CVs and another of period-bounce CVs.

For systems showing Balmer absorption from the white dwarf (WD), hydrogen-dominated atmosphere models were fitted and subtracted to correct for the WD component. H{alpha}, H{beta}, and H{gamma} fluxes were measured. We then investigated statistical relations between the Balmer decrements, the H{beta} line luminosity, and Porb, and compared the measured Balmer decrements with theoretical predictions from accretion disc models.

Short-period pre-bounce CVs show flat Balmer decrements, i.e. Balmer line ratios close to unity, consistent with optically thick line emission and local thermodynamic equilibrium accretion disc models. In contrast, systems near and beyond the period minimum exhibit progressively steeper decrements (H{alpha}/H{beta}>1 and H{gamma}/H{beta}<1). This behaviour is attributed to their lower mass accretion rates.

Balmer decrements can effectively be used to discriminate between short-period pre-bouncers and period-bouncers. The observed steeper Balmer decrements in period-bouncers are consistent with more extended optically thin regions and a more pronounced radial stratification of the physical conditions within their accretion discs. Our logistic-regression classifier based on the H{gamma}/H{beta} and H{alpha}/H{beta} ratios offers a robust and efficient tool for identifying period-bouncer candidates in optical spectroscopic surveys.