We present the discovery of the second binary with a Roche lobe-filling hot subdwarf transferring mass to a white dwarf ( WD ) companion .
This 56 minute binary was discovered using data from the Zwicky Transient Facility .
Spectroscopic observations reveal an He-sdOB star with an effective temperature of T _ { eff } = 33 , 700 \pm 1000 K and a surface gravity of \log ( g ) = 5.54 \pm 0.11 .
The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD .
We infer a He-sdOB mass of M _ { sdOB } = 0.41 \pm 0.04 M _ { \odot } and a WD mass of M _ { WD } = 0.68 \pm 0.05 M _ { \odot } .
The weak eclipses imply a WD black-body temperature of 63 , 000 \pm 10 , 000 K and a radius R _ { WD } = 0.0148 \pm 0.0020 R _ { \odot } as expected for a WD of such high temperature .

The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for \approx 1 Myrs at a rate of 10 ^ { -9 } M _ { \odot } { yr } ^ { -1 } which is consistent with the high WD temperature .
The hot subdwarf will then turn into a WD and the system will merge in \approx 30 Myrs .
We suggest that Galactic reddening could bias discoveries towards preferentially finding Roche lobe-filling systems during the short-lived shell burning phase .
Studies using reddening corrected samples should reveal a large population of helium core-burning hot subdwarfs with T _ { eff } \approx 25 , 000 K in binaries of 60-90 minutes with WDs .
Though not yet in contact , these binaries would eventually come into contact through gravitational wave emission and explode as a sub-luminous thermonuclear supernova or evolve into a massive single WD .