ALMA points to a merger of white dwarf/brown dwarf binary in the 1670s

In the 1980s CK Vul was identified with Nova Vul 1670, as a potential "old nova" that could be used to test the "hibernation" scenario in relation to classical and dwarf novae. Since that time a number of different classifications have been suggested and discarded, including a classical nova, a very-late thermal pulse, a sub-Chandrasekhar mass supernova, a diffusion-induced nova and the ingestion of planets by their host star. Most recently the object has been linked to the red novae, of which V838 Mon is the most prominent member. The behaviour of this class is most likely due to a merger in a binary star system. However the characteristics of the brightening events in the 1670s that led to it being noticed by 17th Century astronomers, and the subsequent evolution, are inconsistent with a merger of any main sequence or giant progenitors. We have reviewed the data to date, and combined that with ALMA Cycle 4 results mapping dusty cocoons, gaseous bubbles, a jet and a central disc-like structure, to propose a new cause of "Nova Vul 1670". We suggest that both the original observations nearly 350 years ago and the current configuration of the system are consistent with a merger between a white dwarf and a brown dwarf. The disruption of the brown dwarf caused the brightening events in 1670, 1671 and 1672. In the process the collision of brown dwarf material with the white dwarf surface caused nucleosynthesis leading to the peculiar isotopic ratios seen in the ejecta. The encounter disrupted the brown dwarf, which formed a disc that persists to this day. The disc rotation and white dwarf magnetic fields generated bipolar jet outflows, which in turn inflated the gaseous bubbles. Over the intervening centuries these have grown to be about 12 arcsec in extent, and are surrounded by dusty cocoons, in a structure reminiscent of asymmetric planetary nebulae and pre-PNe. Further work remains to be done on the gas dynamics, and proposed observations will attempt to measure the central object mass directly from a disc rotation curve.