Informing the Cataclysmic Variable Donor Sequence from Gaia DR2\n Color-Magnitude and Inferred Variability Metrics

Short-period cataclysmic variables (spCVs), with orbital periods below the\nperiod gap ($P_{orb}$ < 2 hr), offer insight into the evolutionary models of\nCVs and can serve as strong emitters of gravitational waves (GWs). To identify\nnew spCV candidates, we crossmatch a catalog of known CVs to sources with\nrobust parallaxes in the Gaia second data release (DR2). We uncover and fit an\napparently monotonic relationship between the color--absolute-magnitude diagram\n(CMD) position and $P_{orb}$ of these CVs, revealed in DR2. To supplement this\nrelation, we develop a method for identifying sources with large photometric\nvariability, a characteristic trait of spCVs. Using all available Gaia light\ncurves, we construct a machine-learned regression model to predict variability\nmetrics for sources in the CMD locus of known spCVs based solely on\ntime-averaged covariates present in DR2. Using this approach we identify 3,253\ncandidate spCVs, of which $\\sim$95% are previously unknown. Inspection of\narchival SDSS spectra of these candidates suggests that $>$82% are likely to be\nspCVs: a noticeably higher recovery rate than previous light curve searches,\nwhich bias toward active systems. We obtain optical spectra of 9 new systems at\nLick Observatory and confirm that all objects are CV systems. We measure\n$P_{orb}$ for 7 systems using archival Gaia and Palomar Transient Factory light\ncurves, 3 of which do not have previous $P_{orb}$ measurements. We use the\nCMD-$P_{orb}$ relation to infer the detectability of these systems to the\nupcoming LISA mission, and find that six sources may be coherent LISA\nverification binaries, with an estimated SNR > 5 in the 4 yr mission. This\npaper demonstrates that the time-averaged Gaia catalog is a powerful tool in\nthe methodical discovery and characterization of time-varying objects, making\nit complementary to missions like ZTF, TESS, and the Vera Rubin LSST.\n