  The LIGO/Virgo collaborations recently announced the detection of a likely
binary neutron star merger, GW190425. The total mass of GW190425 is
significantly larger than the masses of Galactic double neutron stars known
through radio astronomy. This suggests that GW190425 formed differently from
Galactic double neutron stars. We hypothesize that GW190425 formed via unstable
"case BB" mass transfer. According to this hypothesis, the progenitor of
GW190425 was a binary consisting of a neutron star and a ${\sim} 4-5 {M_\odot}$
helium star, which underwent a common-envelope process. Following the
supernovae of the helium star core, a tight, eccentric, double neutron star was
formed, which merged in ${\lesssim}$ 10 Myr. The helium star progenitor may
explain the unusually large mass of GW190425, while the short time to merger
may explain why we do not see similar systems in radio. In order to test this
hypothesis, we measure the eccentricity of GW190425 using publicly available
LIGO/Virgo data. We constrain the eccentricity at 10 Hz to be $e \leq 0.007$
with $90\%$ confidence. This result provides no evidence for or against the
unstable mass transfer scenario because the binary is likely to have
circularized to $e\lesssim10^{-4}$ by the time it entered the LIGO/Virgo band.
Future detectors operating in lower frequency bands will enable us to discern
the formation channel of mergers similar to GW190425 using eccentricity
measurements.
