PTF1 J082340.04+081936.5: A hot subdwarf B star with a low mass white dwarf companion in an 87 minute orbit

We present the discovery of the hot subdwarf B star (sdB) binary PTF1 J082340.04+081936.5. The system has an orbital period P$_{rm orb}=87.49668(1)$ min (0.060761584(10) days), making it the second-most compact sdB binary known. The lightcurve shows ellipsoidal variations. Under the assumption that the sdB primary is synchronized with the orbit, we find a mass $M_{rm sdB}=0.45^{+0.09}_{-0.07}$ M$_odot$, a companion white dwarf mass $M_{rm WD}=0.46^{+0.12}_{-0.09}$ M$_odot$ and a mass ratio $q = frac{M_{rm WD}}{M_{rm sdB}}=1.03^{+0.10}_{-0.08}$. The future evolution was calculated using the MESA stellar evolution code. Adopting a canonical sdB mass of $M_{rm sdB}=0.47$ M$_odot$, we find that the sdB still burns helium at the time it will fill its Roche lobe if the orbital period was less than 106 min at the exit from the last common envelope phase. For longer common envelope exit periods the sdB will have stopped burning helium and turned into a C/O white dwarf at the time of contact. Comparing the spectroscopically derived log(g) and $T_{rm eff}$ with our MESA models, we find that an sdB model with a hydrogen envelope mass of $5times10^{-4} M_odot$ matches the measurements at a post-common envelope age of 94 Myr, corresponding to a post-common envelope orbital period of 109 min which is close to the limit to start accretion while the sdB is still burning helium.