V photometry of Titania, Oberon, and Triton

V filter photometry with the 2.2-m University of Hawaii telescope on Mauna Kea obtained during 1982-1983 is analyzed to determine the phase angle and orbital brightness variations of Titania, Oberon, and Triton. The unit distance opposition magnitudes and phase coefficients (α < 3°) are Titania, V(1,0) = 1.016 ± 0.042, β = 0.102 ± 0.021 mag/deg; Oberon, V(1,0) = 1.231 ± 0.035, β = 0.103 ± 0.018 mag/deg; Triton V(1,0) = -1.236 ± 0.041; β = 0.027 ± 0.035 mag/deg. The phase coefficients for Titania and Oberon are comparable to those observed for asteroids at similar small phase angles. Measurements made at α = 0.°06 show an ≈0.2-mag additional increase in brightness similar to that reported in the near-infrared by R. H. Brown and D. P. Cruikshank ((1983), Icarus 55, 83-92). The small phase coefficient for Triton indicates the light may not be scattered from a low-albedo, porous regolith, but suggests a high-albedo surface, a significant atmosphere, or a smooth surface, e.g., an ocean. Orbital lightcurves are less than 0.1 mag in amplitude for Titania, Oberon, and Triton. The Titania data agree well with photometry at phase angles ≥0.°8 from the Voyager 2 imaging experiment (J. Veverka, P. Thomas, P. Helfenstein, R. H. Brown, and T. V. Johnson, 1987, J. Geophys. Res. 92, 14,895-14,904).