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Neutron star mergers where both stars have negligible spins are commonlyconsidered as the most likely, "standard" case. But based on observed systems,we estimate that actually a non-negligible fraction of all double neutron starmergers ($\sim$ 5 %) may contain one millisecond component. We use theLagrangian Numerical Relativity code SPHINCS_BSSN to simulate mergers where onestar has no spin and the other has a dimensionless spin parameter of$\chi=0.5$. These mergers exhibit several distinct signatures compared toirrotational cases. Morphologically, they are similar to unequal mass mergersand they form in particular only one, very pronounced spiral arm. Compared tothe non-spinning cases, they dynamically eject an order of magnitude more massof unshocked material at the original low electron fraction of the neutronstars and therefore produce particularly bright, red kilonovae and brighterkilonova afterglows months after the merger. We also find that the spinningcases have significantly more fallback accretion, with implications forlate-time X-ray flares and the duration of the associated gamma-ray burst.Overall, the spinning case collisions are substantially less violent and theyemit smaller amounts of shock-generated semi-relativistic material andtherefore produce less pronounced blue/UV kilonova precursor signals. Theirpost-merger gravitational wave signal is weaker and, during the simulated time,substantially smaller amounts of energy and angular momentum are emitted.Therefore the central remnant contains a larger angular momentum reservoir andcould remain an "active engine" for a longer time.

Mergers of double neutron stars with one high-spin component: brighter kilonovae and fallback accretion, weaker gravitational waves