2021ApJ...922..245B

Binary systems of a hot subdwarf B (sdB) star + a white dwarf (WD) with orbital periods less than 2-3 hr can come into contact due to gravitational waves and transfer mass from the sdB star to the WD before the sdB star ceases nuclear burning and contracts to become a WD. Motivated by the growing class of observed systems in this category, we study the phases of mass transfer in these systems. We find that because the residual outer hydrogen envelope accounts for a large fraction of an sdB star's radius, sdB stars can spend a significant amount of time (∼tens of megayears) transferring this small amount of material at low rates (∼10^–10-10^–9 M_☉ yr^–1) before transitioning to a phase where the bulk of their He transfers at much faster rates ( >=10^–8 M_☉ yr^–1). These systems therefore spend a surprising amount of time with Roche-filling sdB donors at orbital periods longer than the range associated with He star models without an envelope. We predict that the envelope transfer phase should be detectable by searching for ellipsoidal modulation of Roche-filling objects with P_orb = 30-100 minutes and T_eff = 20,000-30,000 K, and that many (>=10) such systems may be found in the Galactic plane after accounting for reddening. We also argue that many of these systems may go through a phase of He transfer that matches the signatures of AM CVn systems, and that some AM CVn systems associated with young stellar populations likely descend from this channel.