tokens: alternatively imply incorrect assumptions about the underlying pulsar population properties. Figure 4. Eccentricities of DNSs identified within the LISA band with S/N > 7. Blue and green distributions are designed to match the low- and high-eccentricity DNSs in the MW, respectively. The two black distributions (different line styles) demonstrate the eccentricities expected from two ad hoc models for putative fast-merging DNSs. 4. Discussion and Conclusions Using the MW DNS merger rate of 210 Myr −1 derived from LIGO (Abbott et al. 2019 ), we find that a 4 yr (8 yr) LISA mission will detect on average 240 (330) DNSs. Approximately 25% of those will be “chirping,” allowing for their characterization as DNSs through their chirp mass. The remaining ≈75% will likely be indistinguishable from lower-mass double white dwarfs. Using a more pessimistic rate of 42 Myr −1 based on the Galactic population of DNSs, we find that LISA will detect 46 (65) DNSs for a 4 yr (8 yr) mission. However, the census of MW DNSs is incomplete as Doppler shifting of the radio waves that pulsars emit makes detecting DNSs with orbital periods ≲1 hr extremely challenging. If these systems exist in significant numbers, the nature of GW orbital decay implies that they were formed with similarly short orbital periods. The presence of such fast-merging DNSs can help resolve any difference between the DNS merger rates as determined by LIGO and the galactic population of DNSs. Because LISA bridges the orbital period gap separating DNSs detected with radio waves and by LIGO, it can detect DNSs that form with short orbital periods. As pointed out by Kyutoku et al. ( 2019 ), the sky position of a DNS as measured by LISA may allow for characterization by radio follow-up observations, provided the DNSs contain at least one radio bright pulsar. If, on the other hand, these systems are radio-quiet, the ability to measure orbital eccentricities as small as 10 −2 by LISA affords, perhaps, the only opportunity to measure and characterize the orbits of such systems, discerning between the different formation scenarios. The existence of such a fast-merging channel would have profound implications on the presence of r -process elements in the Universe. Recent studies have shown that, if r -process enrichment seen in the ultra-faint dwarf galaxies Reticulum II and Tucana III is formed from the merger of DNSs, then a fast-merging channel is required (Beniamini Piran 2016 ; Safarzadeh et al. 2019a ). Similar arguments are required to explain the existence of r -process enrichment in globular clusters (Zevin et al. 2019 ). These studies, which use binary population synthesis, suggest that such fast-merging DNSs could be formed through unstable Case BB mass transfer (Delgado Thomas 1981 ), in which a NS accretor enters a second common envelope when its stripped helium star companion evolves into a giant star. Dynamical formation provides an alternative evolutionary scenario that can explain the
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