We compare the critical properties of the two-dimensional (2D) XY model in a transverse magnetic field with filling factors f = 1/3 and 2/5. To obtain a comparison with recent experiments, we investigate the effect of weak quenched bond disorder for f = 2/5. A finite-size scaling analysis of extensive Monte Carlo simulations strongly suggests that the critical exponents of the phase transition for f = 1/3 and for f = 2/5 with disorder, fall into the 2D Ising model universality class. Studying the possible domain walls in the system provides some explanations for our results. 64.70.Rh, 05.70.Fh, 64.60.Fr, 74.50.+r The frustrated XY model provides a convenient framework to study a variety of fascinating phenomena displayed by numerous physical systems. One experimental realization of this model is in two-dimensional arrays of Josephson junctions and superconducting wire networks [1][2][3]. A perpendicular magnetic field induces a finite density of circulating supercurrents, or vortices, within the array. The interplay of two length scales -the mean separation of vortices and the period of the underlying physical array -gives rise to a wide variety of interesting physical phenomena. Many of these effects show up as variations in the properties of the finite-temperature superconducting phase transitions at different fields. Recent and ongoing experiments have measured the critical exponents in superconducting arrays [3], opening the opportunity to do careful comparison of theory and experiment. In this Letter we examine the critical properties of the 2D XY model for two different values of the magnetic field in the densely frustrated regime (f ≫ 0) and in the presence of disorder.The Hamiltonian of the frustrated XY model iswhere θ j is the phase on site j of a square L × L lattice and A ij = (2π/φ 0 ) j i A · dl is the integral of the vector potential from site i to site j with φ 0 being the flux quantum. The directed sum of the A ij around an elementary plaquette A ij = 2πf where f , measured in the units of φ 0 , is the magnetic flux penetrating each plaquette due to the uniformly applied field. We focus here on the cases f = p/q with p/q = 1/3 and 2/5.A unit cell of the ground state fluxoid pattern for these f is shown in Figure 1(a) [4]. The pattern consists of diagonal stripes composed of a single line of vortices for f = 1 3 and a double line of vortices for f = 2 5 . (A vortex is a plaquette with unit fluxoid occupation, ie. the phase gains 2π when going around the plaquette.) The stripes shown in Figure 1(a) can sit on q sub-lattices, which we associate with members of the Z q group. They can also go along either diagonal, and we associate these two options with members of the Z 2 group. A common speculation for commensurate-incommensurate transitions and the frustrated XY model is that the transition should be in the universality class of the q-state (or 2q-state) Pott's model. We find that this is not the case because domain walls between the different states vary considerably in both energetic and ent...