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Binary statistics of the latest-type T and Y brown dwarfs are sparse and itis unclear whether the trends seen in the multiplicity properties of their moremassive counterparts hold for the very coolest brown dwarfs. We present resultsfrom a search for substellar and planetary-mass companions to a sample of 12ultracool T8$-$Y0 field brown dwarfs with the Hubble Space Telescope/Wide FieldCamera 3. We find no evidence for resolved binary companions among our sampledown to separations of 0.7$-$2.5 AU. Combining our survey with prior searches,we place some of the first statistically robust constraints to date on themultiplicity properties of the coolest, lowest-mass brown dwarfs in the field.Accounting for observational biases and incompleteness, we derive a binaryfrequency of $f = 5.5^{+5.2}_{-3.3}$% for T5$-$Y0 brown dwarfs at separationsof 1.5$-$1000 AU, for an overall binary fraction of $f_\mathrm{tot} = 8\pm6$%.Modelling the projected separation as a lognormal distribution, we find a peakin separation at $\rho_0 = 2.9^{+0.8}_{-1.4}$ AU with a logarithmic width of$\sigma = 0.21^{+0.14}_{-0.08}$. We infer a mass ratio distribution peakingstrongly towards unity, with a power law index of $\gamma = 6.1^{+4.0}_{-2.7}$,reinforcing the significance of the detection of a tighter and higher massratio companion population around lower-mass primaries. These results areconsistent with prior studies and support the idea of a decreasing binaryfrequency with spectral type in the Galactic field.

Constraining the multiplicity statistics of the coolest brown dwarfs: binary fraction continues to decrease with spectral type