2021MNRAS.500.5098K

We present the Apache Point Observatory BG40 broad-band and simultaneous Gemini r-band and i-band high-speed follow-up photometry observations and analysis of the 40.5-min period eclipsing detached double-degenerate binary SDSS J082239.54+304857.19. Our APO data spans over 318 d and includes 13 primary eclipses, from which we precisely measure the system's orbital period and improve the time of mid-eclipse measurement. We fit the light curves for each filter individually and show that this system contains a low-mass DA white dwarf with radius R_A = 0.031 ± 0.006 R_☉ and a R_B = 0.013 ± 0.005 R_☉ companion at an inclination of i = 87.7 ± 0.2^*. We use the best-fitting eclipsing light curve model to estimate the temperature of the secondary star as T_eff = 5200 ± 100 K. Finally, while we do not record significant offsets to the expected time of mid-eclipse caused by the emission of gravitational waves with our 1-yr baseline, we show that a 3σ significant measurement of the orbital decay due to gravitational waves will be possible in 2023, at which point the eclipse will occur about 8 s earlier than expected.