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We present LAMOST~J041920.07+072545.4 (hereafter J0419), a close binaryconsisting of a bloated extremely low mass pre-white dwarf (pre-ELM WD) and acompact object with an orbital period of 0.607189~days. The large-amplitudeellipsoidal variations and the evident Balmer and He~I emission lines suggest afilled Roche lobe and ongoing mass transfer. No outburst events were detectedin the 15 years of monitoring of J0419, indicating a very low mass transferrate. The temperature of the pre-ELM, $T_\mathrm{eff} = 5793_{-133}^{+124}\,\rmK$, is cooler than the known ELMs, but hotter than most CV donors. Combiningthe mean density within the Roche lobe and the radius constrained from our SEDfitting, we obtain the mass of the pre-ELM, $M_1 = 0.176\pm 0.014\,M_\odot$.The joint fitting of light and radial velocity curves yields an inclinationangle of $i = 66.5_{-1.7}^{+1.4}$ degrees, corresponding to the compact objectmass of $M_2 = 1.09\pm 0.05\,M_\odot$. The very bloated pre-ELM has a smallersurface gravity ($\log g = 3.9\pm 0.01$, $R_1 = 0.78 \pm 0.02\,R_\odot$) thanthe known ELMs or pre-ELMs. The temperature and the luminosity ($L_\mathrm{bol}= 0.62_{-0.10}^{+0.11}\,L_\odot$) of J0419 are close to the main sequence,which makes the selection of such systems through the HR 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 orbitevolution, which makes J0419 to be a unique source to connect ELM/pre-ELM WDsystems, wide binaries and cataclysmic variables.

A long-period pre-ELM system discovered from LAMOST medium-resolution survey