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We present observations of the Pisces A and B galaxies with the AdvancedCamera for Surveys on the Hubble Space Telescope. Photometry from these imagesclearly resolve a Red Giant Branch for both objects, demonstrating that theyare nearby dwarf galaxies. We describe a Bayesian inferential approach todetermining the distance to these galaxies using the magnitude of the tip ofthe RGB, and then apply this approach to these galaxies. We also provide thefull probability distributions for parameters derived using this approach. Thisreveals the distance to these galaxies as $5.64^{+0.13}_{-0.15} \, {\rm Mpc}$and $8.89^{+0.75}_{-0.85} \, {\rm Mpc}$ for Pisces A and B, respectively,placing both within the Local Volume but not the Local Group. We estimate thestar formation histories of these galaxies, which suggests that they haverecently undergone an increase in their star formation rates. Together theseyield luminosities for Pisces A and B of $M_V=-11.57^{+0.06}_{-0.05}$ and$-12.9 \pm 0.2$, respectively, and estimated stellar masses of$\log(M_*/M_{\odot})= 7.0^{+0.4}_{-1.7}$ and $7.5^{+0.3}_{-1.8}$. We furthershow that these galaxies are likely at the boundary between nearby voids andhigher-density filamentary structure. This suggests that they are entering ahigher-density region from voids, where they would have experienced delayedevolution, consistent with their recent increased star formation rates. If thisis indeed the case, they are useful for study as proxies of the galaxies thatlater evolved into typical Local Group satellite galaxies.

HST Imaging of the Local Volume Dwarf Galaxies Pisces A&B: Prototypes for Local Group Dwarfs