A Systematic Gaia–ZTF Search for Short-period Blue Compact-binary Candidates

Abstract Short-period binaries containing white dwarfs or hot subdwarfs are among the strongest Galactic sources for millihertz gravitational-wave (GW) observatories, but the confirmed sample remains small. Combining Gaia DR3 astrometry and photometry with ZTF DR23 light curves—through a Gaia selection calibrated against known systems, ZTF period searches, and machine learning morphology ranking—we assemble a catalog of 147 short-period (10.34–106.46 minutes) blue compact-binary candidates, 111 of them without a previous compact-binary classification. DESI DR1, GALEX, and AllWISE data reveal an intrinsically heterogeneous sample: on the Gaia color–magnitude diagram it comprises 52 white-dwarf-locus, 69 hot-subdwarf-region, and 26 intermediate sources, and among the 26 members with DESI spectra only about one-third lie on the white-dwarf cooling sequence, the rest being more luminous blue stars whose low-resolution spectra resemble white dwarfs. We identify a prioritized set of new short-period white-dwarf-locus candidates for time-resolved follow-up, including 10 with periods below 40 minutes and none with an existing radial-velocity series. Under fiducial binary assumptions, 17 newly identified white-dwarf-locus candidates would exceed the adopted LISA threshold, led by a 37 pc white dwarf; no candidate has a measured chirp mass, and with the signal-to-noise ratio scaling as M c 5 / 3 the count is 9 for M c = 0.15 M ⊙ , 17 for the adopted 0.3 M ⊙ , and 21 for 0.6 M ⊙ , in each case assuming the modulation is orbital. We show, however, that for the compact majority of the white-dwarf-locus sample the observed modulation amplitudes exceed any possible ellipsoidal signal by 3 to 5 orders of magnitude, so a rotating magnetic or chemically inhomogeneous single white dwarf is a competing interpretation that ZTF photometry alone cannot exclude; the catalog contains at least one confirmed case. We release the full 147-source catalog with periods, Gaia and spectroscopic classifications, harmonic and ellipsoidal diagnostics, and supplementary tables of fiducial GW estimates and ultraviolet–infrared photometry.