Images, color–color, and color–magnitude diagrams of the fields around the late O-type stars BD+42 3760, J20325571+4307583, and J20462826+4223417. The O stars are the brightest stars slightly left of the center of the images. North is at the top and east to the left in all the images, and the field of view measures approximately . The thick dots in the color–color and color–magnitude diagrams represent intrinsically blue stars. These stars are characterized by a value of the reddening-free index Q = (U − B) − 0.72(B − V) < −0.7. The solid lines represent the locus of the unreddened main sequence at a distance modulus DM = 10.8. The dotted lines are the extinction vectors that have their origin at the position of a B0V star, with a length corresponding to an extinction A_V = 10 mag. The uncertainties in the photometry of the faintest stars plotted in each diagram are noted in the lower left.