Detailed follow up studies of three ultracompact sdB binaries

Hot subdwarf stars are thought to be helium-core burning stars which lost most of their hydrogen rich envelope. Most of these stars are found in compact binaries with a sub-sample showing orbital periods well below one hour. These ultracompact systems have white dwarf companions and as such they are double detonation type Ia supernova progenitors if the white dwarfs are sufficiently massive. We present follow-up studies of three ultracompact hot subdwarf binaries. Using data from the Zwicky Transient Facility, we find find orbital periods of 33.6, 37.3, and 36.9 minutes for ZTF 1946+3203, ZTF 0640+1738, and ZTF 0643+0318 respectively. The light curves show ellipsoidal variability of the hot subdwarf star with potential eclipses of an accretion disc. Phase-resolved spectroscopic observations with Keck were used to measure a radial velocity curve and atmospheric parameters of the hot subdwarf stars. ZTF J0643 shows evidence of accretion disc emission lines in the average spectrum. Combining light curve and spectroscopic fits will allow us to measure precise system properties such as masses, to determine the evolutionary and future evolution of these systems. In a preliminary analysis for ZTF 1946+3203 we find a mass-transferring hot subdwarf star with a low mass white dwarf companion. This system is below the Chandrasekhar mass, so the white dwarf will most likely merge with the SdB companion, forming a single R CrB star instead of exploding.