ZTF confirms Gaia DR3 ellipsoidal amplitudes of 126 compact-companion candidates, but 40 have shapes outside a dark-companion Roche model and 29 of 30 with good parallax overfill the Roche lobe

Gaia DR3 lists 6,306 ellipsoidal variables that can have dark, massive companions. For 262 of them the one-sigma lower bound of the modified minimum mass ratio (mMMR) is more than 1. We fitted ZTF DR24 g and r light curves of 739 catalogue sources at the Gaia period: 126 high-confidence sources, 604 random comparison sources, and 14 systems that radial velocities already showed to have low-mass companions (Nagarajan et al. 2023). ZTF gives the Gaia double-wave period for 120 of 126 high-confidence sources, and the ZTF r to Gaia G amplitude ratio has a median of 1.01. Thus the large amplitudes are real and are not a Gaia sampling effect; with ZTF amplitudes, 117 of 126 keep mMMR > 1. We calculated 7,776 light curves per band of a Roche-distorted star with a dark point-mass companion (q = 0.5 to 20). In these models the cos(4 phi)/cos(2 phi) ratio is never above 0.086. The high-confidence group has an error-weighted mean ratio of 0.106 +/- 0.002, and 40 of 126 sources fail at least one shape test (2-sigma ratio rule or joint chi-square p < 0.001). Gaia GSP-Phot distances for these stars are about half the parallax distances, so we used parallax radii. If the companion is dark, 96% of 995 catalogue stars with parallax/error > 5 are larger than their Roche lobes (median R/R_L = 1.55); a conservative variant (lowest radius over all extinctions up to the SFD value, parallax +2 sigma, mass x1.5) gives 15%. The radius test flags 12 of 13 RV-rejected systems, but the shape test flags only 2 of 14, so a model-consistent shape is not evidence for a compact companion. The results agree with contact or near-contact binaries of two luminous stars for most candidates. One high-confidence source (Gaia DR3 1827494570016143232) passes both best-estimate tests at low parallax precision. Limitations: ZTF covers only declination > -30 deg, few candidates have precise parallaxes, the radius test depends on extinction and mass, and the Roche model has no spots.

Gaia DR3 · ZTF · ellipsoidal variables · compact objects · black hole candidates · neutron star candidates · contact binaries · Roche model · false positives · vari_compact_companion