A Long-period Pre-ELM System Discovered from the LAMOST Medium-resolution Survey

We present LAMOST J041920.07+072545.4 (hereafter J0419), a close binary consisting of a bloated, extremely low mass pre-white dwarf (pre-ELM WD) and a compact object with an orbital period of 0.607189 days. The large-amplitude ellipsoidal variations and the evident Balmer and He I emission lines suggest a filled Roche lobe and ongoing mass transfer. No outburst events were detected in the 15 years of monitoring of J0419, indicating a very low mass transfer rate. The temperature of the pre-ELM, T-eff = 5793(-133)(+124) K, makes it cooler than the known ELMs, but hotter than most cataclysmic variable donors. Combining the mean density within the Roche lobe and the radius constrained from our spectral energy distribution fitting, we obtain the mass of the pre-ELM, M-1 = 0.176 +/- 0.014 M-circle dot. The joint fitting of light and radial velocity curves yields an inclination angle of i = 66.5(-1.7)(+1.4) degrees, corresponding to a mass of the compact object of M-2 = 1.09 +/- 0.05 M-circle dot. The very bloated pre-ELM has a smaller surface gravity (log g = 3.9 +/- 0.01, R-1 = 0.78 +/- 0.02 R-circle dot) than the known ELMs or pre-ELMs. The temperature and the luminosity (L-bol = 0.62 L-+0.11(-0.10)circle dot) of J0419 are close to those of the main sequence, which makes the selection of such systems through the H-R diagram inefficient. Based on the evolutionary model, the relatively long period and small log g indicate that J0419 could be close to the "bifurcation period" in the orbital evolution, which makes J0419 a unique source to connect ELM/pre-ELM WD systems, wide binaries, and cataclysmic variables.