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AM CVn systems are ultra-compact binaries where a white dwarf accretes from ahelium-rich degenerate or semi-degenerate donor. Some AM CVn systems will beamong the loudest sources of gravitational waves for the upcoming LaserInterferometer Space Antenna (LISA), yet the formation channel of AM CVnsremains uncertain. We report the study and characterisation of a new eclipsingAM CVn, SRGeJ045359.9+622444 (hereafter SRGeJ0453), discovered from a jointSRG/eROSITA and ZTF program to identify cataclysmic variables (CVs). Weobtained optical photometry to confirm the eclipse of SRGeJ0453 and determinethe orbital period to be $P_\textrm{orb} = 55.0802 \pm 0.0003$ min. Weconstrain the binary parameters by modeling the high-speed photometry andradial velocity curves and find $M_\textrm{donor} = 0.044 \pm0.024 M_{\odot}$and $R_\textrm{donor}=0.078 \pm 0.012 R_{\odot}$. The X-ray spectrum isapproximated by a power-law model with an unusually flat photon index of$\Gamma\sim 1$ previously seen in magnetic CVs with SRG/eROSITA, but verifyingthe magnetic nature of SRGeJ0453 requires further investigation. Opticalspectroscopy suggests that the donor star of SRGeJ0453 could have initiallybeen a He star or a He white dwarf. SRGeJ0453 is the ninth eclipsing AM CVnsystem published to date, and its lack of optical outbursts have made itelusive in previous surveys. The discovery of SRGeJ0453 using joint X-ray andoptical surveys highlights the potential for discovering similar systems in thenear future.

SRGeJ045359.9+622444: A 55-min Period Eclipsing AM CVn Discovered from a Joint SRG/eROSITA + ZTF Search